diff --git a/src/libfetchers/github.cc b/src/libfetchers/github.cc
index 2e914164a..ecfd035fc 100644
--- a/src/libfetchers/github.cc
+++ b/src/libfetchers/github.cc
@@ -261,11 +261,12 @@ struct GitArchiveInputScheme : InputScheme
         auto tarballCache = getTarballCache();
         auto parseSink = tarballCache->getFileSystemObjectSink();
         auto lastModified = unpackTarfileToSink(archive, *parseSink);
+        auto tree = parseSink->sync();
 
         act.reset();
 
         TarballInfo tarballInfo {
-            .treeHash = tarballCache->dereferenceSingletonDirectory(parseSink->sync()),
+            .treeHash = tarballCache->dereferenceSingletonDirectory(tree),
             .lastModified = lastModified
         };
 
diff --git a/src/libfetchers/tarball.cc b/src/libfetchers/tarball.cc
index dd4f3b780..a082b0078 100644
--- a/src/libfetchers/tarball.cc
+++ b/src/libfetchers/tarball.cc
@@ -170,6 +170,7 @@ static DownloadTarballResult downloadTarball_(
     auto tarballCache = getTarballCache();
     auto parseSink = tarballCache->getFileSystemObjectSink();
     auto lastModified = unpackTarfileToSink(archive, *parseSink);
+    auto tree = parseSink->sync();
 
     act.reset();
 
@@ -184,7 +185,7 @@ static DownloadTarballResult downloadTarball_(
     } else {
         infoAttrs.insert_or_assign("etag", res->etag);
         infoAttrs.insert_or_assign("treeHash",
-            tarballCache->dereferenceSingletonDirectory(parseSink->sync()).gitRev());
+            tarballCache->dereferenceSingletonDirectory(tree).gitRev());
         infoAttrs.insert_or_assign("lastModified", uint64_t(lastModified));
         if (res->immutableUrl)
             infoAttrs.insert_or_assign("immutableUrl", *res->immutableUrl);