diff --git a/src/libfetchers/fetchers.cc b/src/libfetchers/fetchers.cc
index 26229134d..9717533d6 100644
--- a/src/libfetchers/fetchers.cc
+++ b/src/libfetchers/fetchers.cc
@@ -220,8 +220,8 @@ void InputScheme::checkLocks(const Input & specified, const Input & final) const
 
     if (auto prevLastModified = specified.getLastModified()) {
         if (final.getLastModified() != prevLastModified)
-            throw Error("'lastModified' attribute mismatch in input '%s', expected %d",
-                final.to_string(), *prevLastModified);
+            throw Error("'lastModified' attribute mismatch in input '%s', expected %d, got %d",
+                final.to_string(), *prevLastModified, final.getLastModified().value_or(-1));
     }
 
     if (auto prevRev = specified.getRev()) {
diff --git a/src/libfetchers/tarball.cc b/src/libfetchers/tarball.cc
index 27ad89b6e..e723d3061 100644
--- a/src/libfetchers/tarball.cc
+++ b/src/libfetchers/tarball.cc
@@ -384,11 +384,13 @@ struct TarballInputScheme : CurlInputScheme
             input = immutableInput;
         }
 
-        /* If we got a lastModified and the input is not final and
-           doesn't have one, then return it. Note that we don't do
-           this if the input is final for compatibility with old lock
-           files that didn't include lastModified. */
-        if (result.lastModified && !_input.isFinal() && !input.attrs.contains("lastModified"))
+        /* If we got a lastModified, then return it. But for
+           compatibility with old lock files that didn't include
+           lastModified, don't do this if the original input was final
+           and didn't contain a lastModified. */
+        if (result.lastModified
+            && !input.attrs.contains("lastModified")
+            && (!_input.isFinal() || _input.attrs.contains("lastModified")))
             input.attrs.insert_or_assign("lastModified", uint64_t(result.lastModified));
 
         input.attrs.insert_or_assign("narHash",