diff --git a/src/nix/prefetch.cc b/src/nix/prefetch.cc
index 55b8498ef5d..c6e60a53be6 100644
--- a/src/nix/prefetch.cc
+++ b/src/nix/prefetch.cc
@@ -118,7 +118,7 @@ std::tuple<StorePath, Hash> prefetchFile(
             /* If the archive unpacks to a single file/directory, then use
                that as the top-level. */
             tmpFile = entries->path();
-            unsigned fileCount = std::distance(entries, std::filesystem::directory_iterator{});
+            auto fileCount = std::distance(entries, std::filesystem::directory_iterator{});
             if (fileCount != 1) {
                 /* otherwise, use the directory itself */
                 tmpFile = unpacked;