die_errno("git commit-tree: failed to read");
        }
 
-       if (commit_tree(&buffer, tree_sha1, parents, commit_sha1,
-                       NULL, sign_commit)) {
+       if (commit_tree(buffer.buf, buffer.len, tree_sha1, parents,
+                       commit_sha1, NULL, sign_commit)) {
                strbuf_release(&buffer);
                return 1;
        }