diff --git a/flax/core/lift.py b/flax/core/lift.py
index f99c9cb64..649f1da29 100644
--- a/flax/core/lift.py
+++ b/flax/core/lift.py
@@ -1491,6 +1491,8 @@ def rematted(variable_groups, rng_groups, *args, **kwargs):
 
 def _hashable_filter(x):
   """Hashable version of CollectionFilter."""
+  if isinstance(x, str):
+    return (x,)
   if isinstance(x, Iterable):
     return tuple(x)  # convert un-hashable list & sets to tuple
   if isinstance(x, DenyList):