diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 93dfd21cce..e94c6b7939 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1332,7 +1332,8 @@ void DerivationGoal::buildDone()
                 StringSink sink;
                 dumpPath(path, sink);
                 deletePath(path);
-                StringSource source(rewriteHashes(sink.s, rewritesFromTmp));
+                sink.s = rewriteHashes(sink.s, rewritesFromTmp);
+                StringSource source(sink.s);
                 restorePath(path, source);
             }