diff --git a/include/sqlpp11/expression.h b/include/sqlpp11/expression.h
index 22c6c92b..73b6c3a8 100644
--- a/include/sqlpp11/expression.h
+++ b/include/sqlpp11/expression.h
@@ -165,7 +165,7 @@ namespace sqlpp
 				context << "(";
 				if (trivial_value_is_null_t<Rhs>::value)
 				{
-					serialize(t._lhs, context);
+					serialize(t._rhs, context);
 					context << " IS NULL ";
 				}
 				else
@@ -174,6 +174,8 @@ namespace sqlpp
 					serialize(t._rhs, context);
 				}
 				context << ")";
+
+				return context;
 			}
 		};
 
diff --git a/tests/MockDb.h b/tests/MockDb.h
index 3356c214..64cbb883 100644
--- a/tests/MockDb.h
+++ b/tests/MockDb.h
@@ -30,7 +30,6 @@
 #include <sqlpp11/serializer_context.h>
 #include <sqlpp11/connection.h>
 
-#warning add serialization to run methods to increase the number of tests
 template<bool enforceNullResultTreatment>
 struct MockDbT: public sqlpp::connection
 {
@@ -105,24 +104,32 @@ struct MockDbT: public sqlpp::connection
 	template<typename Insert>
 		size_t insert(const Insert& x)
 		{
+			_serializer_context_t context;
+			::sqlpp::serialize(x, context);
 			return 0;
 		}
 
 	template<typename Update>
 		size_t update(const Update& x)
 		{
+			_serializer_context_t context;
+			::sqlpp::serialize(x, context);
 			return 0;
 		}
 
 	template<typename Remove>
 		size_t remove(const Remove& x)
 		{
+			_serializer_context_t context;
+			::sqlpp::serialize(x, context);
 			return 0;
 		}
 
 	template<typename Select>
-		result_t select(const Select& s)
+		result_t select(const Select& x)
 		{
+			_serializer_context_t context;
+			::sqlpp::serialize(x, context);
 			return {};
 		}
 
@@ -139,24 +146,32 @@ struct MockDbT: public sqlpp::connection
 	template<typename Insert>
 		_prepared_statement_t prepare_insert(Insert& x)
 		{
+			_serializer_context_t context;
+			::sqlpp::serialize(x, context);
 			return nullptr;
 		}
 
 	template<typename PreparedInsert>
 		size_t run_prepared_insert(const PreparedInsert& x)
 		{
+			_serializer_context_t context;
+			::sqlpp::serialize(x, context);
 			return 0;
 		}
 
 	template<typename Select>
 		_prepared_statement_t prepare_select(Select& x)
 		{
+			_serializer_context_t context;
+			::sqlpp::serialize(x, context);
 			return nullptr;
 		}
 
 	template<typename PreparedSelect>
 		result_t run_prepared_select(PreparedSelect& x)
 		{
+			_serializer_context_t context;
+			::sqlpp::serialize(x, context);
 			return {};
 		}