diff --git a/src/gt4py/next/ffront/past_to_itir.py b/src/gt4py/next/ffront/past_to_itir.py
index a5d266c4c0..0fc9a6280d 100644
--- a/src/gt4py/next/ffront/past_to_itir.py
+++ b/src/gt4py/next/ffront/past_to_itir.py
@@ -71,11 +71,11 @@ class Meta:
         model = PastToItir
 
 
-def _column_axis(all_closure_vars: dict[str, Any]) -> common.Dimension:
+def _column_axis(all_closure_vars: dict[str, Any]) -> Optional[common.Dimension]:
     # construct mapping from column axis to scan operators defined on
     #  that dimension. only one column axis is allowed, but we can use
     #  this mapping to provide good error messages.
-    scanops_per_axis: dict[common.Dimension, str] = {}
+    scanops_per_axis: dict[common.Dimension, list[str]] = {}
     for name, gt_callable in transform_utils._filter_closure_vars_by_type(
         all_closure_vars, gtcallable.GTCallable
     ).items():