diff --git a/src/libfetchers/tarball.cc b/src/libfetchers/tarball.cc
index 2538ab8c2..ab61a8e8b 100644
--- a/src/libfetchers/tarball.cc
+++ b/src/libfetchers/tarball.cc
@@ -312,7 +312,7 @@ struct TarballInputScheme : CurlInputScheme
             input = immutableInput;
         }
 
-        if (result.lastModified && !input.attrs.contains("lastModified"))
+        if (result.lastModified && !input.attrs.count("lastModified"))
             input.attrs.insert_or_assign("lastModified", uint64_t(result.lastModified));
 
         return {result.tree.storePath, std::move(input)};