diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index a685b13..83921ea 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -268,7 +268,8 @@
             if (rwa) {
             bad_mode:
                 PyErr_SetString(PyExc_ValueError,
-                                "Must have exactly one of read/write/append mode");
+                                "Must have exactly one of read/write/append "
+                                "mode and at most one plus");
                 goto error;
             }
             rwa = 1;