diff --git a/src/restic/backend/test/tests.go b/src/restic/backend/test/tests.go
index 4519ee25c..65b7d8978 100644
--- a/src/restic/backend/test/tests.go
+++ b/src/restic/backend/test/tests.go
@@ -277,7 +277,8 @@ func BackendTestLoad(t testing.TB, s *Suite) {
 
 type errorCloser struct {
 	io.Reader
-	t testing.TB
+	size int64
+	t    testing.TB
 }
 
 func (ec errorCloser) Close() error {
@@ -285,6 +286,10 @@ func (ec errorCloser) Close() error {
 	return errors.New("forbidden method close was called")
 }
 
+func (ec errorCloser) Size() int64 {
+	return ec.size
+}
+
 // BackendTestSave tests saving data in the backend.
 func BackendTestSave(t testing.TB, s *Suite) {
 	b := s.open(t)
@@ -354,7 +359,7 @@ func BackendTestSave(t testing.TB, s *Suite) {
 
 	// wrap the tempfile in an errorCloser, so we can detect if the backend
 	// closes the reader
-	err = b.Save(h, errorCloser{t: t, Reader: tmpfile})
+	err = b.Save(h, errorCloser{t: t, size: int64(length), Reader: tmpfile})
 	if err != nil {
 		t.Fatal(err)
 	}