diff --git a/mercurial/cext/dirs.c b/mercurial/cext/dirs.c
--- a/mercurial/cext/dirs.c
+++ b/mercurial/cext/dirs.c
@@ -66,6 +66,11 @@
 	while ((pos = _finddir(cpath, pos - 1)) != -1) {
 		PyObject *val;
 
+		// Sniff for trailing slashes, a marker of an invalid input.
+		if (cpath[pos] == '/') {
+			goto bail;
+		}
+
 		key = PyBytes_FromStringAndSize(cpath, pos);
 		if (key == NULL)
 			goto bail;