diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index 4d502219e..93519ec66 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -652,6 +652,7 @@ Path Downloader::downloadCached(ref<Store> store, const string & url_, bool unpa
                 Hash hash = hashString(expectedHash ? expectedHash.type : htSHA256, *res.data);
                 info.path = store->makeFixedOutputPath(false, hash, name);
                 info.narHash = hashString(htSHA256, *sink.s);
+                info.narSize = sink.s->size();
                 info.ca = makeFixedOutputCA(false, hash);
                 store->addToStore(info, sink.s, false, true);
                 storePath = info.path;
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index c8e61126c..ed03c3f42 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -919,8 +919,12 @@ void LocalStore::addToStore(const ValidPathInfo & info, const ref<std::string> &
 
     Hash h = hashString(htSHA256, *nar);
     if (h != info.narHash)
-        throw Error(format("hash mismatch importing path ‘%s’; expected hash ‘%s’, got ‘%s’") %
-            info.path % info.narHash.to_string() % h.to_string());
+        throw Error("hash mismatch importing path ‘%s’; expected hash ‘%s’, got ‘%s’",
+            info.path, info.narHash.to_string(), h.to_string());
+
+    if (nar->size() != info.narSize)
+        throw Error("szie mismatch importing path ‘%s’; expected %s, got %s",
+            info.path, info.narSize, nar->size());
 
     if (requireSigs && !dontCheckSigs && !info.checkSignatures(*this, publicKeys))
         throw Error("cannot add path ‘%s’ because it lacks a valid signature", info.path);