diff --git a/Documentation/technical/api-lockfile.txt b/Documentation/technical/api-lockfile.txt
index 99830f3bf3..6538610759 100644
--- a/Documentation/technical/api-lockfile.txt
+++ b/Documentation/technical/api-lockfile.txt
@@ -147,7 +147,8 @@ commit_lock_file::
 	`hold_lock_file_for_append`, close the file descriptor and
 	rename the lockfile to its final destination. Return 0 upon
 	success or a negative value on failure to `close(2)` or
-	`rename(2)`.
+	`rename(2)`. It is a bug to call `commit_lock_file()` for a
+	`lock_file` object that is not currently locked.
 
 rollback_lock_file::
 
diff --git a/lockfile.c b/lockfile.c
index e148227fb1..c897dd8a93 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -301,6 +301,9 @@ int commit_lock_file(struct lock_file *lk)
 {
 	char result_file[PATH_MAX];
 
+	if (!lk->filename[0])
+		die("BUG: attempt to commit unlocked object");
+
 	if (close_lock_file(lk))
 		return -1;