diff --git a/lib/gen_valid.py b/lib/gen_valid.py
index 36a63db..e23bde1 100755
--- a/lib/gen_valid.py
+++ b/lib/gen_valid.py
@@ -230,6 +230,14 @@
         error_message += gp.sprint_var(valid_values)
         return process_error_message(error_message)
 
+    error_message = valid_type(valid_values, list, var_name='valid_values')
+    if error_message:
+        return process_error_message(error_message)
+
+    error_message = valid_type(invalid_values, list, var_name='invalid_values')
+    if error_message:
+        return process_error_message(error_message)
+
     if len_valid_values > 0:
         # Processing the valid_values list.
         if var_value in valid_values: