diff --git a/src/input/InputStream.hxx b/src/input/InputStream.hxx
index 25e99de3d..c62f71e45 100644
--- a/src/input/InputStream.hxx
+++ b/src/input/InputStream.hxx
@@ -249,7 +249,7 @@ public:
 	gcc_pure
 	offset_type GetRest() const {
 		assert(ready);
-		assert(size >= 0);
+		assert(KnownSize());
 		assert(offset >= 0);
 
 		return size - offset;