diff --git a/runtime/doc/testing.txt b/runtime/doc/testing.txt
index 15c5498..74b705c 100644
--- a/runtime/doc/testing.txt
+++ b/runtime/doc/testing.txt
@@ -291,8 +291,9 @@
 			catch
 			  call assert_exception('E492:')
 			endtry
-
-assert_fails({cmd} [, {error} [, {msg}]])			*assert_fails()*
+<
+							*assert_fails()*
+assert_fails({cmd} [, {error} [, {msg} [, {lnum}]]])
 		Run {cmd} and add an error message to |v:errors| if it does
 		NOT produce an error or when {error} is not found in the
 		error message.  Also see |assert-return|.
@@ -311,13 +312,21 @@
 		string for the first error: >
 			assert_fails('cmd', ['', 'E987:'])
 <
+		If {msg} is empty then it is not used.  Do this to get the
+		default message when passing the {lnum} argument.
+
+		When {lnum} is present and not negative, and the {error}
+		argument is present and matches, then this is compared with
+		the line number at which the error was reported. That can be
+		the line number in a function or in a script.
+
 		Note that beeping is not considered an error, and some failing
 		commands only beep.  Use |assert_beeps()| for those.
 
 		Can also be used as a |method|: >
 			GetCmd()->assert_fails('E99:')
 
-assert_false({actual} [, {msg}])				*assert_false()*
+assert_false({actual} [, {msg}])			*assert_false()*
 		When {actual} is not false an error message is added to
 		|v:errors|, like with |assert_equal()|.
 		Also see |assert-return|.