diff --git a/main/src/com/google/refine/expr/functions/ToDate.java b/main/src/com/google/refine/expr/functions/ToDate.java
index b74655cfe..c748be0dc 100644
--- a/main/src/com/google/refine/expr/functions/ToDate.java
+++ b/main/src/com/google/refine/expr/functions/ToDate.java
@@ -50,29 +50,30 @@ import com.google.refine.expr.EvalError;
 import com.google.refine.expr.util.CalendarParser;
 import com.google.refine.expr.util.CalendarParserException;
 import com.google.refine.grel.Function;
+import com.google.refine.grel.ControlFunctionRegistry;
 import com.google.refine.util.ParsingUtilities;
 
 public class ToDate implements Function {
 
     @Override
     public Object call(Properties bindings, Object[] args) {
-        if (args.length == 0) {
-            // missing value, can this happen?
-            return null;
-        }
-        Object arg0 = args[0];
         String o1;
-        if (arg0 instanceof Date) {
-            return arg0;
-        } else if (arg0 instanceof Calendar) {
-            return ((Calendar) arg0).getTime();
-        } else if (arg0 instanceof Long) {
-            o1 =  ((Long) arg0).toString(); // treat integers as years
-        } else if (arg0 instanceof String) {
-            o1 = (String) arg0;
+        if (args.length == 0) {
+            return new EvalError(ControlFunctionRegistry.getFunctionName(this) + " expects at least one argument");
         } else {
-            // ignore cell values that aren't strings
-            return new EvalError("Not a String - cannot parse to date");
+            Object arg0 = args[0];
+            if (arg0 instanceof Date) {
+                return arg0;
+            } else if (arg0 instanceof Calendar) {
+                return ((Calendar) arg0).getTime();
+            } else if (arg0 instanceof Long) {
+                o1 =  ((Long) arg0).toString(); // treat integers as years
+            } else if (arg0 instanceof String && arg0.toString().trim().length() > 0) {
+                o1 = (String) arg0;
+            } else {
+                // ignore cell values that aren't Date, Calendar, Long or String 
+                return new EvalError("Unable to parse as date");
+            }
         }
 
         // "o, boolean month_first (optional)"
@@ -98,13 +99,12 @@ public class ToDate implements Function {
 //                    } catch (DatatypeConfigurationException e2) {     
 //                    }
                 }
-                return new EvalError("Cannot parse to date");
+                return new EvalError("Unable to parse as date");
             }
-        }
-
-        // "o, format1, format2 (optional), ..."
-        Locale locale = Locale.getDefault();
-        if (args.length>=2) {
+        } else if (args.length>=2) {
+            // "o, format1, format2 (optional), ..."
+            Locale locale = Locale.getDefault();
+        
             for (int i=1;i<args.length;i++) {
                 if (!(args[i] instanceof String)) {
                     // skip formats that aren't strings
@@ -156,9 +156,9 @@ public class ToDate implements Function {
                 }
             }
             return new EvalError("Unable to parse as date");
+        } else {
+            return new EvalError("Unable to parse as date");
         }
-
-        return null;
     }
 
 
diff --git a/main/src/com/google/refine/expr/functions/ToNumber.java b/main/src/com/google/refine/expr/functions/ToNumber.java
index b0cdfb041..4935b3199 100644
--- a/main/src/com/google/refine/expr/functions/ToNumber.java
+++ b/main/src/com/google/refine/expr/functions/ToNumber.java
@@ -39,6 +39,7 @@ import org.json.JSONException;
 import org.json.JSONWriter;
 
 import com.google.refine.expr.EvalError;
+import com.google.refine.grel.ControlFunctionRegistry;
 import com.google.refine.grel.Function;
 
 public class ToNumber implements Function {
@@ -58,12 +59,15 @@ public class ToNumber implements Function {
                     try {
                         return Double.parseDouble(s);
                     } catch (NumberFormatException e) {
-                        return new EvalError("Cannot parse to number");
+                        return new EvalError("Unable to parse as number");
                     }
+                } else {
+                    return new EvalError("Unable to parse as number");
                 }
             }
+        } else {
+            return new EvalError(ControlFunctionRegistry.getFunctionName(this) + " expects one non-null argument");
         }
-        return null;
     }
     
     @Override
diff --git a/main/src/com/google/refine/expr/functions/ToString.java b/main/src/com/google/refine/expr/functions/ToString.java
index 2a3b54b82..0a03ae169 100644
--- a/main/src/com/google/refine/expr/functions/ToString.java
+++ b/main/src/com/google/refine/expr/functions/ToString.java
@@ -43,6 +43,7 @@ import org.json.JSONException;
 import org.json.JSONWriter;
 
 import com.google.refine.expr.EvalError;
+import com.google.refine.grel.ControlFunctionRegistry;
 import com.google.refine.grel.Function;
 import com.google.refine.util.StringUtils;
 
@@ -68,7 +69,7 @@ public class ToString implements Function {
                 } 
             }
         }
-        return new EvalError("ToString accepts an object and an optional second argument containing a date format string");
+        return new EvalError(ControlFunctionRegistry.getFunctionName(this) + " accepts an object and an optional second argument containing a date format string");
     }
 
     
diff --git a/main/tests/server/src/com/google/refine/tests/expr/functions/strings/ToFromConversionTests.java b/main/tests/server/src/com/google/refine/tests/expr/functions/strings/ToFromConversionTests.java
index fbc6d9a9e..5db608229 100644
--- a/main/tests/server/src/com/google/refine/tests/expr/functions/strings/ToFromConversionTests.java
+++ b/main/tests/server/src/com/google/refine/tests/expr/functions/strings/ToFromConversionTests.java
@@ -98,10 +98,9 @@ public class ToFromConversionTests extends RefineTest {
     
     @Test
     public void testToNumber() {
-//        Assert.assertTrue(invoke("toNumber") instanceof EvalError);
-        Assert.assertNull(invoke("toNumber"));
-//        Assert.assertTrue(invoke("toNumber", (Object) null) instanceof EvalError);
-        Assert.assertNull(invoke("toNumber", (Object) null));
+        Assert.assertTrue(invoke("toNumber") instanceof EvalError);
+        Assert.assertTrue(invoke("toNumber", (Object) null) instanceof EvalError);
+        Assert.assertTrue(invoke("toNumber", "") instanceof EvalError);
         Assert.assertTrue(invoke("toNumber", "string") instanceof EvalError);
         Assert.assertEquals(invoke("toNumber", "0.0"), 0.0);
         Assert.assertEquals(invoke("toNumber", "123"), Long.valueOf(123));
@@ -128,9 +127,9 @@ public class ToFromConversionTests extends RefineTest {
     
     @Test
     public void testToDate() throws CalendarParserException {
-//      Assert.assertTrue(invoke("toDate") instanceof EvalError);
-      Assert.assertNull(invoke("toDate"));
+      Assert.assertTrue(invoke("toDate") instanceof EvalError);
       Assert.assertTrue(invoke("toDate", (Object) null) instanceof EvalError);
+      Assert.assertTrue(invoke("toDate", "") instanceof EvalError);
       Assert.assertTrue(invoke("toDate", 1.0) instanceof EvalError);
       Assert.assertTrue(invoke("toDate", "2012-03-01","xxx") instanceof EvalError); // bad format string
       Assert.assertTrue(invoke("toDate", "2012-03-01") instanceof GregorianCalendar);