diff --git a/src/pygit2/error.c b/src/pygit2/error.c
index 6e94336..bb806d1 100644
--- a/src/pygit2/error.c
+++ b/src/pygit2/error.c
@@ -113,6 +113,9 @@ PyObject* Error_set_str(int err, const char *str)
     }
 
     error = giterr_last();
+    if (error == NULL) //exptected error - no error msg set
+        return PyErr_Format(Error_type(err), "%s", str);
+
     return PyErr_Format(Error_type(err), "%s: %s", str, error->message);
 }