diff --git a/artiq/compiler/transforms/inferencer.py b/artiq/compiler/transforms/inferencer.py
index e5cbc561b..e2be2c7df 100644
--- a/artiq/compiler/transforms/inferencer.py
+++ b/artiq/compiler/transforms/inferencer.py
@@ -888,7 +888,7 @@ class Inferencer(algorithm.Visitor):
                         break
                     if types.is_var(elt):
                         return  # undetermined yet
-                    if not builtins.is_iterable(elt):
+                    if not builtins.is_iterable(elt) or builtins.is_str(elt):
                         break
                     num_dims += 1
                     elt = builtins.get_iterable_elt(elt)