give non-iterable TypeError a message (closes #20507)
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 09e41b9..29f9bce 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -5618,7 +5618,7 @@
     PyObject *item;
     Py_ssize_t i;
 
-    fseq = PySequence_Fast(seq, "");
+    fseq = PySequence_Fast(seq, "can only join an iterable");
     if (fseq == NULL) {
         return NULL;
     }