diff --git a/borg/_chunker.c b/borg/_chunker.c
index 70d611650..87ec31576 100644
--- a/borg/_chunker.c
+++ b/borg/_chunker.c
@@ -152,6 +152,7 @@ chunker_fill(Chunker *c)
         }
         else {
             // some error happened
+            PyErr_SetFromErrno(PyExc_OSError);
             return 0;
         }
         length = c->bytes_read - offset;