diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index fe7bf69..4051650 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -486,6 +486,15 @@
   CheckScriptFailure(['def Func(arg: number)', 'arg = 3', 'enddef', 'defcompile'], 'E1090:')
 enddef
 
+" These argument names are reserved in legacy functions.
+def WithReservedNames(firstline: string, lastline: string): string
+  return firstline .. lastline
+enddef
+
+def Test_argument_names()
+  assert_equal('OK', WithReservedNames('O', 'K'))
+enddef
+
 def Test_call_func_defined_later()
   g:DefinedLater('one')->assert_equal('one')
   assert_fails('NotDefined("one")', 'E117:', '', 2, 'Test_call_func_defined_later')
diff --git a/src/userfunc.c b/src/userfunc.c
index 595963d..be2a18c 100644
--- a/src/userfunc.c
+++ b/src/userfunc.c
@@ -66,8 +66,9 @@
     while (ASCII_ISALNUM(*p) || *p == '_')
 	++p;
     if (arg == p || isdigit(*arg)
-	    || (p - arg == 9 && STRNCMP(arg, "firstline", 9) == 0)
-	    || (p - arg == 8 && STRNCMP(arg, "lastline", 8) == 0))
+	    || (argtypes == NULL
+		&& ((p - arg == 9 && STRNCMP(arg, "firstline", 9) == 0)
+		    || (p - arg == 8 && STRNCMP(arg, "lastline", 8) == 0))))
     {
 	if (!skip)
 	    semsg(_("E125: Illegal argument: %s"), arg);
diff --git a/src/version.c b/src/version.c
index dd1934d..2344ddf 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1718,
+/**/
     1717,
 /**/
     1716,