diff --git a/runtime/doc/builtin.txt b/runtime/doc/builtin.txt
index 2f6961d..6a5ec1b 100644
--- a/runtime/doc/builtin.txt
+++ b/runtime/doc/builtin.txt
@@ -156,7 +156,7 @@
 echoraw({expr})			none	output {expr} as-is
 empty({expr})			Number	|TRUE| if {expr} is empty
 environ()			Dict	return environment variables
-err_teapot()			Number	produce error 418
+err_teapot([{expr}])		none	give E418, or E503 if {expr} is |TRUE|
 escape({string}, {chars})	String	escape {chars} in {string} with '\'
 eval({string})			any	evaluate {string} into its value
 eventhandler()			Number	|TRUE| if inside an event handler