diff --git a/include/musique.hh b/include/musique.hh
index c422de5..5d926c7 100644
--- a/include/musique.hh
+++ b/include/musique.hh
@@ -457,7 +457,7 @@ struct Lines
 	void add_file(std::string filename, std::string_view source);
 
 	/// Add single line into file (REPL usage)
-	void add_line(std::string const& filename, std::string_view source);
+	void add_line(std::string const& filename, std::string_view source, unsigned line_number);
 
 	/// Print selected region
 	void print(std::ostream& os, std::string const& file, unsigned first_line, unsigned last_line) const;
@@ -604,7 +604,7 @@ struct Parser
 
 	/// Parses whole source code producing Ast or Error
 	/// using Parser structure internally
-	static Result<Ast> parse(std::string_view source, std::string_view filename);
+	static Result<Ast> parse(std::string_view source, std::string_view filename, unsigned line_number = 0);
 
 	/// Parse sequence, collection of expressions
 	Result<Ast> parse_sequence();
diff --git a/src/errors.cc b/src/errors.cc
index 31fa5d5..35ae955 100644
--- a/src/errors.cc
+++ b/src/errors.cc
@@ -312,7 +312,12 @@ std::ostream& operator<<(std::ostream& os, Error const& err)
 				}
 			}
 		},
-		[&](errors::Undefined_Operator const&)               { unimplemented(); },
+		[&](errors::Undefined_Operator const& err) {
+			os << "I encountered '" << err.op <<  "' that looks like operator but it isn't one.\n";
+			os << '\n';
+
+			print_error_line(loc);
+		},
 		[&](errors::Unexpected_Keyword const&)               { unimplemented(); },
 	}, err.details);
 
diff --git a/src/lines.cc b/src/lines.cc
index 5da416d..d6d74d4 100644
--- a/src/lines.cc
+++ b/src/lines.cc
@@ -21,9 +21,12 @@ void Lines::add_file(std::string filename, std::string_view source)
 	}
 }
 
-void Lines::add_line(std::string const& filename, std::string_view source)
+void Lines::add_line(std::string const& filename, std::string_view source, unsigned line_number)
 {
-	lines[filename].push_back(source);
+	assert(line_number != 0, "Line number = 0 is invalid");
+	if (lines[filename].size() <= line_number)
+		lines[filename].resize(line_number);
+	lines[filename][line_number - 1] = source;
 }
 
 void Lines::print(std::ostream &os, std::string const& filename, unsigned first_line, unsigned last_line) const
diff --git a/src/main.cc b/src/main.cc
index d460335..c7d39c9 100644
--- a/src/main.cc
+++ b/src/main.cc
@@ -17,6 +17,7 @@ namespace fs = std::filesystem;
 
 static bool ast_only_mode = false;
 static bool enable_repl = false;
+static unsigned repl_line_number = 1;
 
 #define Ignore(Call) do { auto const ignore_ ## __LINE__ = (Call); (void) ignore_ ## __LINE__; } while(0)
 
@@ -150,7 +151,7 @@ struct Runner
 	/// Run given source
 	Result<void> run(std::string_view source, std::string_view filename, bool output = false)
 	{
-		auto ast = Try(Parser::parse(source, filename));
+		auto ast = Try(Parser::parse(source, filename, repl_line_number));
 
 		if (ast_only_mode) {
 			dump(ast);
@@ -264,8 +265,9 @@ static Result<void> Main(std::span<char const*> args)
 
 	for (auto const& [is_file, argument] : runnables) {
 		if (!is_file) {
-			Lines::the.add_line("<arguments>", argument);
+			Lines::the.add_line("<arguments>", argument, repl_line_number);
 			Try(runner.run(argument, "<arguments>"));
+			repl_line_number++;
 			continue;
 		}
 		auto path = argument;
@@ -286,6 +288,7 @@ static Result<void> Main(std::span<char const*> args)
 	}
 
 	if (runnables.empty() || enable_repl) {
+		repl_line_number = 1;
 		enable_repl = true;
 		bestlineSetCompletionCallback(completion);
 		for (;;) {
@@ -317,12 +320,13 @@ static Result<void> Main(std::span<char const*> args)
 				continue;
 			}
 
-			Lines::the.add_line("<repl>", raw);
+			Lines::the.add_line("<repl>", raw, repl_line_number);
 			auto result = runner.run(raw, "<repl>", true);
 			if (not result.has_value()) {
 				std::cout << std::flush;
 				std::cerr << result.error() << std::flush;
 			}
+			repl_line_number++;
 			// We don't free input line since there could be values that still relay on it
 		}
 	}
diff --git a/src/parser.cc b/src/parser.cc
index 09b7b0a..2f55069 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -44,10 +44,11 @@ static Result<std::vector<Ast>> parse_many(
 	std::optional<Token::Type> separator,
 	At_Least at_least);
 
-Result<Ast> Parser::parse(std::string_view source, std::string_view filename)
+Result<Ast> Parser::parse(std::string_view source, std::string_view filename, unsigned line_number)
 {
 	Lexer lexer{source};
 	lexer.location.filename = filename;
+	lexer.location.line = line_number;
 	Parser parser;
 
 	for (;;) {