diff --git a/pystencils/astnodes.py b/pystencils/astnodes.py
index d9867fed68748e47cb56e40a8ba0f8b013ba6a05..e3dca67d218ba1e5c1cfc9c586b8450fe926c341 100644
--- a/pystencils/astnodes.py
+++ b/pystencils/astnodes.py
@@ -517,7 +517,7 @@ class SympyAssignment(Node):
 
     @property
     def undefined_symbols(self):
-        result = self.rhs.atoms(sp.Symbol)
+        result = {s for s in self.rhs.free_symbols if not isinstance(s, sp.Indexed)}
         # Add loop counters if there a field accesses
         loop_counters = set()
         for symbol in result: