--- a/spec/util_format_spec.lua	Fri Dec 10 22:48:45 2021 +0100
+++ b/spec/util_format_spec.lua	Sat Dec 11 13:30:34 2021 +0100
@@ -18,6 +18,7 @@
 
 		it("escapes ascii control stuff", function ()
 			assert.equal("␁", format("%s", "\1"));
+			assert.equal("[␁]", format("%d", "\1"));
 		end);
 
 		it("escapes invalid UTF-8", function ()