diff --git a/src/cmd/go/internal/modfetch/fetch.go b/src/cmd/go/internal/modfetch/fetch.go
index 61759c0e6b..fd7a5cef83 100644
--- a/src/cmd/go/internal/modfetch/fetch.go
+++ b/src/cmd/go/internal/modfetch/fetch.go
@@ -514,6 +514,7 @@ func checkModSum(mod module.Version, h string) error {
 	goSum.mu.Lock()
 	inited, err := initGoSum()
 	if err != nil {
+		goSum.mu.Unlock()
 		return err
 	}
 	done := inited && haveModSumLocked(mod, h)