diff --git a/extensions/wikidata/module/scripts/warningsrenderer.js b/extensions/wikidata/module/scripts/warningsrenderer.js
index cf788c46f..120ef1f0e 100644
--- a/extensions/wikidata/module/scripts/warningsrenderer.js
+++ b/extensions/wikidata/module/scripts/warningsrenderer.js
@@ -15,10 +15,15 @@ WarningsRenderer._renderEntity = function(entity) {
       fullLabel = entity.label + ' (' + id + ')';
   }
 
+  var url = entity.iri;
+  if (!url && entity.value) {
+     url = 'http://www.wikidata.org/entity/'+entity.value.id;
+  }
+
   if (is_new) {
      return '<span class="wb-preview-new-entity">'+fullLabel+'</span>';
   } else {
-     return '<a href="'+entity.iri+'" class="wb-preview-entity" target="_blank">'+fullLabel+'</a>';
+     return '<a href="'+url+'" class="wb-preview-entity" target="_blank">'+fullLabel+'</a>';
   }
 }
 
diff --git a/extensions/wikidata/src/org/openrefine/wikidata/qa/EditInspector.java b/extensions/wikidata/src/org/openrefine/wikidata/qa/EditInspector.java
index b90bb3101..8a74bb3c4 100644
--- a/extensions/wikidata/src/org/openrefine/wikidata/qa/EditInspector.java
+++ b/extensions/wikidata/src/org/openrefine/wikidata/qa/EditInspector.java
@@ -74,6 +74,7 @@ public class EditInspector {
         // so that all newly created entities appear in the batch
         WikibaseAPIUpdateScheduler scheduler = new WikibaseAPIUpdateScheduler();
         editBatch = scheduler.schedule(editBatch);
+
         Map<EntityIdValue, ItemUpdate> updates =  ItemUpdate.groupBySubject(editBatch);
         List<ItemUpdate> mergedUpdates = updates.values().stream().collect(Collectors.toList());
         for(EditScrutinizer scrutinizer : scrutinizers.values()) {
diff --git a/extensions/wikidata/src/org/openrefine/wikidata/schema/ExpressionContext.java b/extensions/wikidata/src/org/openrefine/wikidata/schema/ExpressionContext.java
index 471c50ee0..71109a02a 100644
--- a/extensions/wikidata/src/org/openrefine/wikidata/schema/ExpressionContext.java
+++ b/extensions/wikidata/src/org/openrefine/wikidata/schema/ExpressionContext.java
@@ -1,9 +1,11 @@
 package org.openrefine.wikidata.schema;
 
+import org.apache.commons.lang.Validate;
 import org.openrefine.wikidata.qa.QAWarning;
 import org.openrefine.wikidata.qa.QAWarningStore;
 
 import com.google.refine.model.Cell;
+import com.google.refine.model.Column;
 import com.google.refine.model.ColumnModel;
 import com.google.refine.model.Row;
 
@@ -11,7 +13,7 @@ import com.google.refine.model.Row;
  * A class holding all the necessary information about
  * the context in which a schema expression is evaluated.
  * 
- * @author antonin
+ * @author Antonin Delpeuch
  *
  */
 public class ExpressionContext {
@@ -23,12 +25,17 @@ public class ExpressionContext {
     
     /**
      * Builds an expression context to evaluate a schema on a row
-     * @param baseIRI: the siteIRI of the schema
-     * @param rowId: the id of the row currently visited
-     * @param row: the row itself
-     * @param columnModel: lets us access cells by column name
-     * @param warningStore: where to store the issues encountered when
-     * evaluating (can be set to null if these issues should be ignored)
+     * @param baseIRI
+     *      the siteIRI of the schema
+     * @param rowId
+     *      the id of the row currently visited
+     * @param row
+     *      the row itself
+     * @param columnModel
+     *      lets us access cells by column name
+     * @param warningStore
+     *      where to store the issues encountered when
+     *      evaluating (can be set to null if these issues should be ignored)
      */
     public ExpressionContext(
             String baseIRI,
@@ -36,9 +43,12 @@ public class ExpressionContext {
             Row row,
             ColumnModel columnModel,
             QAWarningStore warningStore) {
+        Validate.notNull(baseIRI);
         this.baseIRI = baseIRI;
         this.rowId = rowId;
+        Validate.notNull(row);
         this.row = row;
+        Validate.notNull(columnModel);
         this.columnModel = columnModel;
         this.warningStore = warningStore;
     }
@@ -46,14 +56,23 @@ public class ExpressionContext {
     public String getBaseIRI() {
         return baseIRI;
     }
-    
-    public int getCellIndexByName(String name) {
-        return columnModel.getColumnByName(name).getCellIndex();
-    }
-    
+    /**
+     * Retrieves a cell in the current row, by column name.
+     * If the column does not exist, null is returned.
+     * 
+     * @param name
+     *     the name of the column to retrieve the cell from
+     * @return
+     *     the cell
+     */
     public Cell getCellByName(String name) {
-        int idx = getCellIndexByName(name);
-        return row.getCell(idx);
+        Column column = columnModel.getColumnByName(name);
+        if (column != null) {
+            int idx = column.getCellIndex();
+            return row.getCell(idx);
+        } else {
+            return null;
+        }
     }
     
     public int getRowId() {
diff --git a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbItemDocumentExpr.java b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbItemDocumentExpr.java
index 6ce9c21a4..7529603ef 100644
--- a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbItemDocumentExpr.java
+++ b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbItemDocumentExpr.java
@@ -1,7 +1,9 @@
 package org.openrefine.wikidata.schema;
 
+import java.util.Collections;
 import java.util.List;
 
+import org.jsoup.helper.Validate;
 import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
 import org.openrefine.wikidata.updates.ItemUpdate;
 import org.openrefine.wikidata.updates.ItemUpdateBuilder;
@@ -34,8 +36,15 @@ public class WbItemDocumentExpr extends JacksonJsonizable implements WbExpressio
             @JsonProperty("subject") WbExpression<? extends ItemIdValue> subjectExpr,
             @JsonProperty("nameDescs") List<WbNameDescExpr> nameDescExprs,
             @JsonProperty("statementGroups") List<WbStatementGroupExpr> statementGroupExprs) {
+        Validate.notNull(subjectExpr);
         this.subject = subjectExpr;
+        if(nameDescExprs == null) {
+            nameDescExprs = Collections.emptyList();
+        }
         this.nameDescs = nameDescExprs;
+        if(statementGroupExprs == null) {
+            statementGroupExprs = Collections.emptyList();
+        }
         this.statementGroups = statementGroupExprs;
     }
     
diff --git a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbMonolingualExpr.java b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbMonolingualExpr.java
index b3b352e99..f5d41d5d0 100644
--- a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbMonolingualExpr.java
+++ b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbMonolingualExpr.java
@@ -1,5 +1,6 @@
 package org.openrefine.wikidata.schema;
 
+import org.apache.commons.lang.Validate;
 import org.openrefine.wikidata.qa.QAWarning;
 import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
 import org.wikidata.wdtk.datamodel.helpers.Datamodel;
@@ -19,7 +20,9 @@ public class WbMonolingualExpr implements WbExpression<MonolingualTextValue> {
     public WbMonolingualExpr(
             @JsonProperty("language") WbExpression<? extends String> languageExpr,
             @JsonProperty("value") WbExpression<? extends StringValue> valueExpr) {
+        Validate.notNull(languageExpr);
         this.languageExpr = languageExpr;
+        Validate.notNull(valueExpr);
         this.valueExpr = valueExpr;
     }
 
diff --git a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbPropConstant.java b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbPropConstant.java
index 8856a45cc..7d1ebf399 100644
--- a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbPropConstant.java
+++ b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbPropConstant.java
@@ -1,5 +1,6 @@
 package org.openrefine.wikidata.schema;
 
+import org.jsoup.helper.Validate;
 import org.openrefine.wikidata.schema.entityvalues.SuggestedPropertyIdValue;
 import org.wikidata.wdtk.datamodel.interfaces.PropertyIdValue;
 
@@ -23,7 +24,9 @@ public class WbPropConstant implements WbExpression<PropertyIdValue> {
             @JsonProperty("pid") String pid,
             @JsonProperty("label") String label,
             @JsonProperty("datatype") String datatype) {
+        Validate.notNull(pid);
         this.pid = pid;
+        Validate.notNull(label);
         this.label = label;
         this.datatype = datatype;
     }
diff --git a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbReferenceExpr.java b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbReferenceExpr.java
index 8344a9305..8f760b2f6 100644
--- a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbReferenceExpr.java
+++ b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbReferenceExpr.java
@@ -3,6 +3,7 @@ package org.openrefine.wikidata.schema;
 import java.util.ArrayList;
 import java.util.List;
 
+import org.apache.commons.lang.Validate;
 import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
 import org.wikidata.wdtk.datamodel.helpers.Datamodel;
 import org.wikidata.wdtk.datamodel.interfaces.Reference;
@@ -28,6 +29,7 @@ public class WbReferenceExpr implements WbExpression<Reference> {
     @JsonCreator
     public WbReferenceExpr(
             @JsonProperty("snaks") List<WbSnakExpr> snakExprs) {
+        Validate.notNull(snakExprs);
         this.snakExprs = snakExprs;
     }
     
diff --git a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbSnakExpr.java b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbSnakExpr.java
index efd20e8ff..0ae3c9c08 100644
--- a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbSnakExpr.java
+++ b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbSnakExpr.java
@@ -1,5 +1,6 @@
 package org.openrefine.wikidata.schema;
 
+import org.jsoup.helper.Validate;
 import org.openrefine.wikidata.schema.exceptions.SkipSchemaExpressionException;
 import org.wikidata.wdtk.datamodel.helpers.Datamodel;
 import org.wikidata.wdtk.datamodel.interfaces.PropertyIdValue;
@@ -28,7 +29,9 @@ public class WbSnakExpr implements WbExpression<Snak> {
     public WbSnakExpr(
             @JsonProperty("prop") WbExpression<? extends PropertyIdValue> propExpr,
             @JsonProperty("value") WbExpression<? extends Value> valueExpr) {
+        Validate.notNull(propExpr);
         this.prop = propExpr;
+        Validate.notNull(valueExpr);
         this.value = valueExpr;
     }
 
diff --git a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbStringConstant.java b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbStringConstant.java
index 67d17f016..e8fade299 100644
--- a/extensions/wikidata/src/org/openrefine/wikidata/schema/WbStringConstant.java
+++ b/extensions/wikidata/src/org/openrefine/wikidata/schema/WbStringConstant.java
@@ -1,5 +1,6 @@
 package org.openrefine.wikidata.schema;
 
+import org.apache.commons.lang.Validate;
 import org.wikidata.wdtk.datamodel.helpers.Datamodel;
 import org.wikidata.wdtk.datamodel.interfaces.StringValue;
 
@@ -13,6 +14,7 @@ public class WbStringConstant implements WbExpression<StringValue> {
     
     @JsonCreator
     public WbStringConstant(@JsonProperty("value") String value) {
+        Validate.notNull(value);
         this.value = value;
     }
     
diff --git a/extensions/wikidata/src/org/openrefine/wikidata/schema/entityvalues/SuggestedEntityIdValue.java b/extensions/wikidata/src/org/openrefine/wikidata/schema/entityvalues/SuggestedEntityIdValue.java
index 4b06a72e6..a0f7d9bfd 100644
--- a/extensions/wikidata/src/org/openrefine/wikidata/schema/entityvalues/SuggestedEntityIdValue.java
+++ b/extensions/wikidata/src/org/openrefine/wikidata/schema/entityvalues/SuggestedEntityIdValue.java
@@ -4,6 +4,7 @@ import java.util.ArrayList;
 import java.util.List;
 
 import org.wikidata.wdtk.datamodel.helpers.Hash;
+import org.wikidata.wdtk.datamodel.helpers.ToString;
 import org.wikidata.wdtk.datamodel.interfaces.EntityIdValue;
 import org.wikidata.wdtk.datamodel.interfaces.ValueVisitor;
 
@@ -70,4 +71,5 @@ public abstract class SuggestedEntityIdValue implements PrefetchedEntityIdValue
     public int hashCode() {
         return Hash.hashCode(this);
     }
+
 }
diff --git a/extensions/wikidata/src/org/openrefine/wikidata/schema/entityvalues/SuggestedPropertyIdValue.java b/extensions/wikidata/src/org/openrefine/wikidata/schema/entityvalues/SuggestedPropertyIdValue.java
index ef072dc3c..c0b8a513b 100644
--- a/extensions/wikidata/src/org/openrefine/wikidata/schema/entityvalues/SuggestedPropertyIdValue.java
+++ b/extensions/wikidata/src/org/openrefine/wikidata/schema/entityvalues/SuggestedPropertyIdValue.java
@@ -1,5 +1,6 @@
 package org.openrefine.wikidata.schema.entityvalues;
 
+import org.wikidata.wdtk.datamodel.helpers.ToString;
 import org.wikidata.wdtk.datamodel.interfaces.PropertyIdValue;
 
 public class SuggestedPropertyIdValue extends SuggestedEntityIdValue implements PropertyIdValue {
@@ -13,4 +14,8 @@ public class SuggestedPropertyIdValue extends SuggestedEntityIdValue implements
         return ET_PROPERTY;
     }
 
+    @Override
+    public String toString() {
+        return "suggested "+ToString.toString(this)+" (\""+getLabel()+"\")";
+    }
 }
diff --git a/extensions/wikidata/tests/src/org/openrefine/wikidata/schema/ExpressionContextTest.java b/extensions/wikidata/tests/src/org/openrefine/wikidata/schema/ExpressionContextTest.java
new file mode 100644
index 000000000..dd71e53ce
--- /dev/null
+++ b/extensions/wikidata/tests/src/org/openrefine/wikidata/schema/ExpressionContextTest.java
@@ -0,0 +1,38 @@
+package org.openrefine.wikidata.schema;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+
+import org.testng.annotations.BeforeMethod;
+import org.testng.annotations.Test;
+
+import com.google.refine.model.Project;
+import com.google.refine.tests.RefineTest;
+
+public class ExpressionContextTest extends RefineTest {
+    
+    Project project = null;
+    
+    @BeforeMethod
+    public void setUp() {
+        project = createCSVProject("a,b\nc\nd,e");
+    }
+    
+    @Test
+    public void testGetCellByColumnName() {
+        ExpressionContext ctxt = new ExpressionContext("foo:", 1, project.rows.get(1), project.columnModel, null);
+        assertEquals("e", ctxt.getCellByName("b").value);
+    }
+    
+    @Test
+    public void testNonExistentColumn() {
+        ExpressionContext ctxt = new ExpressionContext("foo:", 1, project.rows.get(1), project.columnModel, null);
+        assertNull(ctxt.getCellByName("auie"));
+    }
+    
+    @Test
+    public void testGetRowId() {
+        ExpressionContext ctxt = new ExpressionContext("foo:", 1, project.rows.get(1), project.columnModel, null);
+        assertEquals(1, ctxt.getRowId());
+    }
+}