diff --git a/deepdiff/diff.py b/deepdiff/diff.py
index 4dfec50..da2ea8e 100755
--- a/deepdiff/diff.py
+++ b/deepdiff/diff.py
@@ -530,7 +530,7 @@ def _get_clean_to_keys_mapping(self, keys, level):
                 clean_key = KEY_TO_VAL_STR.format(type_, clean_key)
             else:
                 clean_key = key
-            if self.ignore_string_case:
+            if self.ignore_string_case and isinstance(clean_key, str):
                 clean_key = clean_key.lower()
             if clean_key in result:
                 logger.warning(('{} and {} in {} become the same key when ignore_numeric_type_changes'