diff --git a/builtin/diff.c b/builtin/diff.c
index 4b46e394ce..18b1083e98 100644
--- a/builtin/diff.c
+++ b/builtin/diff.c

@@ -579,9 +579,13 @@ int cmd_diff(int argc,
 		obj = deref_tag(the_repository, obj, NULL, 0);
 		if (!obj)
 			die(_("invalid object '%s' given."), name);
-		if (obj->type == OBJ_COMMIT)
-			obj = &repo_get_commit_tree(the_repository,
-						    ((struct commit *)obj))->object;
+		if (obj->type == OBJ_COMMIT) {
+			struct tree *tree = repo_get_commit_tree(
+				the_repository, (struct commit *)obj);
+			if (!tree)
+				die(_("unable to read tree object for commit '%s'"), name);
+			obj = &tree->object;
+		}