diff --git a/borg/_chunker.c b/borg/_chunker.c
index 20461e7c6..4db21b75b 100644
--- a/borg/_chunker.c
+++ b/borg/_chunker.c
@@ -127,7 +127,7 @@ chunker_free(Chunker *c)
 static int
 chunker_fill(Chunker *c)
 {
-    size_t n;
+    ssize_t n;
     PyObject *data;
     memmove(c->data, c->data + c->last, c->position + c->remaining - c->last);
     c->position -= c->last;