diff --git a/Python/pythonrun.c b/Python/pythonrun.c
index 38db290..977ee21 100644
--- a/Python/pythonrun.c
+++ b/Python/pythonrun.c
@@ -895,7 +895,7 @@
 {
 	PyObject *m, *d, *v;
 	const char *ext;
-	int set_file_name = 0, ret;
+	int set_file_name = 0, ret, len;
 
 	m = PyImport_AddModule("__main__");
 	if (m == NULL)
@@ -912,7 +912,8 @@
 		set_file_name = 1;
 		Py_DECREF(f);
 	}
-	ext = filename + strlen(filename) - 4;
+	len = strlen(filename);
+	ext = filename + len - (len > 4 ? 4 : 0);
 	if (maybe_pyc_file(fp, filename, ext, closeit)) {
 		/* Try to run a pyc file. First, re-open in binary */
 		if (closeit)