diff --git a/src/awkward/_attrs.py b/src/awkward/_attrs.py
index e0e837eb73..1a904ba9a9 100644
--- a/src/awkward/_attrs.py
+++ b/src/awkward/_attrs.py
@@ -76,10 +76,10 @@ def to_dict(self):
         return _unfreeze_attrs(self._data)
 
 
-def _enforce_str_key(attr: Any) -> str:
-    if not isinstance(attr, str):
-        raise TypeError(f"'attrs' keys must be strings: {attr!r}")
-    return attr
+def _enforce_str_key(key: Any) -> str:
+    if not isinstance(key, str):
+        raise TypeError(f"'attrs' keys must be strings, got: {key!r}")
+    return key
 
 
 def _freeze_attrs(attrs: Mapping[str, Any]) -> Mapping[str, Any]:
diff --git a/tests/test_3350_enforce_attrs_string_keys.py b/tests/test_3350_enforce_attrs_string_keys.py
new file mode 100644
index 0000000000..20c1123c18
--- /dev/null
+++ b/tests/test_3350_enforce_attrs_string_keys.py
@@ -0,0 +1,17 @@
+# BSD 3-Clause License; see https://github.com/scikit-hep/awkward/blob/main/LICENSE
+# ruff: noqa: E402
+
+from __future__ import annotations
+
+import pytest
+import awkward as ak
+
+
+def test():
+    arr = ak.Array([1])
+
+    with pytest.raises(
+        TypeError,
+        match="'attrs' keys must be strings, got: 1",
+    ):
+        arr.attrs[1] = "foo"