give non-iterable TypeError a message (closes #20507)

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index e1ff999e136980c23c6a9cc723d2f0f5278c2141..89094e000f91f15296ad7c4875ce58fadb9b5070 100644 (file)
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -9493,7 +9493,7 @@ PyUnicode_Join(PyObject *separator, PyObject *seq)
     PyObject *last_obj;
     unsigned int kind = 0;
 
-    fseq = PySequence_Fast(seq, "");
+    fseq = PySequence_Fast(seq, "can only join an iterable");
     if (fseq == NULL) {
         return NULL;
     }