diff --git a/pystencils/transformations.py b/pystencils/transformations.py
index 6fd80687bd36c401303b7c8c215a5be3a64a774c..fc09f34e439c51c57b6579781619522eb166b510 100644
--- a/pystencils/transformations.py
+++ b/pystencils/transformations.py
@@ -576,8 +576,8 @@ def move_constants_before_loop(ast_node):
             if isinstance(element, ast.Conditional):
                 break
             else:
-                critical_symbols = element.symbols_defined
-            if node.undefined_symbols.intersection(critical_symbols):
+                critical_symbols = set([s.name for s in element.symbols_defined])
+            if set([s.name for s in node.undefined_symbols]).intersection(critical_symbols):
                 break
             prev_element = element
             element = element.parent