diff --git a/symmetric-io/src/main/java/org/jumpmind/symmetric/io/data/writer/TransformWriter.java b/symmetric-io/src/main/java/org/jumpmind/symmetric/io/data/writer/TransformWriter.java
index 5421f695a3..e3ecdd5df6 100644
--- a/symmetric-io/src/main/java/org/jumpmind/symmetric/io/data/writer/TransformWriter.java
+++ b/symmetric-io/src/main/java/org/jumpmind/symmetric/io/data/writer/TransformWriter.java
@@ -308,8 +308,15 @@ protected boolean perform(DataContext context, TransformedData data,
                                     data.put(transformColumn,
                                             ((NewAndOldValue) value).getNewValue(),
                                             ((NewAndOldValue) value).getOldValue(), false);
-                                } else {
+                                } else if (value == null || value instanceof String) {
                                     data.put(transformColumn, (String) value, null, false);
+                                } else if (value instanceof List) {
+                                    throw new IllegalStateException(String.format("Column transform failed %s.%s. Transforms that multiply rows must be marked as part of the primary key", 
+                                            transformColumn.getTransformId(), transformColumn.getTargetColumnName()));                                    
+                                } else {                                    
+                                    throw new IllegalStateException(String.format("Column transform failed %s.%s. It returned an unexpected type of %s", 
+                                            transformColumn.getTransformId(), transformColumn.getTargetColumnName(), 
+                                            value.getClass().getSimpleName()));
                                 }
                             } catch (IgnoreColumnException e) {
                                 // Do nothing. We are ignoring the column