diff --git a/objstore.go b/objstore.go
index 116538ca..5bce3ef6 100644
--- a/objstore.go
+++ b/objstore.go
@@ -837,5 +837,9 @@ type ObjectSizerReadCloser struct {
 
 // ObjectSize implement ObjectSizer.
 func (o ObjectSizerReadCloser) ObjectSize() (int64, error) {
+	if o.Size == nil {
+		return 0, errors.New("unknown size")
+	}
+
 	return o.Size()
 }