diff --git a/mercurial/cext/parsers.c b/mercurial/cext/parsers.c
--- a/mercurial/cext/parsers.c
+++ b/mercurial/cext/parsers.c
@@ -487,9 +487,11 @@
    to make sure it is correct. */
 static PyObject *dirstate_item_set_possibly_dirty(dirstateItemObject *self)
 {
-	if (self->flags |= dirstate_flag_possibly_dirty) {
+	self->flags |= dirstate_flag_possibly_dirty;
+	if (self->flags) {
 		Py_RETURN_NONE;
 	}
+	return NULL;
 }
 
 /* See docstring of the python implementation for details */