diff --git a/t/README b/t/README
index 0e4e8d8862..fecb76e599 100644
--- a/t/README
+++ b/t/README
@@ -275,6 +275,14 @@ library for your script to use.
    Merges the given rev using the given message.  Like test_commit,
    creates a tag and calls test_tick before committing.
 
+ - test_must_fail <git-command>
+
+   Run a git command and ensure it fails in a controlled way.  Use
+   this instead of "! <git-command>".  When git-command dies due to a
+   segfault, test_must_fail diagnoses it as an error; "! <git-command>"
+   treats it as just another expected failure, which would let such a
+   bug go unnoticed.
+
 Tips for Writing Tests
 ----------------------