diff --git a/src/structs.h b/src/structs.h
index d6f4988..be4082b 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1610,6 +1610,8 @@
     int		uf_dfunc_idx;	// only valid if uf_def_status is UF_COMPILED
     garray_T	uf_args;	// arguments, including optional arguments
     garray_T	uf_def_args;	// default argument expressions
+    int		uf_args_visible; // normally uf_args.ga_len, less when
+				 // compiling default argument expression.
 
     // for :def (for :function uf_ret_type is NULL)
     type_T	**uf_arg_types;	// argument types (count == uf_args.ga_len)
diff --git a/src/testdir/test_vim9_func.vim b/src/testdir/test_vim9_func.vim
index 4816cb6..f4142ee 100644
--- a/src/testdir/test_vim9_func.vim
+++ b/src/testdir/test_vim9_func.vim
@@ -452,6 +452,12 @@
       MyDefaultThird('->', 'xx', v:none)->assert_equal('->xxbb')
       MyDefaultThird('->', v:none, 'yy')->assert_equal('->aayy')
       MyDefaultThird('->', 'xx', 'yy')->assert_equal('->xxyy')
+
+      def DefArg(mandatory: any, optional = mandatory): string
+        return mandatory .. optional
+      enddef
+      DefArg(1234)->assert_equal('12341234')
+      DefArg("ok")->assert_equal('okok')
   END
   CheckDefAndScriptSuccess(lines)
 
diff --git a/src/version.c b/src/version.c
index 0cefeba..5f07483 100644
--- a/src/version.c
+++ b/src/version.c
@@ -756,6 +756,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3091,
+/**/
     3090,
 /**/
     3089,
diff --git a/src/vim9compile.c b/src/vim9compile.c
index d074215..88086ce 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -274,7 +274,7 @@
 
     if (len == 0)
 	return FAIL;
-    for (idx = 0; idx < cctx->ctx_ufunc->uf_args.ga_len; ++idx)
+    for (idx = 0; idx < cctx->ctx_ufunc->uf_args_visible; ++idx)
     {
 	char_u *arg = FUNCARG(cctx->ctx_ufunc, idx);
 
@@ -9172,7 +9172,6 @@
     {
 	int	count = ufunc->uf_def_args.ga_len;
 	int	first_def_arg = ufunc->uf_args.ga_len - count;
-	int	uf_args_len = ufunc->uf_args.ga_len;
 	int	i;
 	char_u	*arg;
 	int	off = STACK_FRAME_SIZE + (ufunc->uf_va_name != NULL ? 1 : 0);
@@ -9195,12 +9194,11 @@
 		goto erret;
 
 	    // Make sure later arguments are not found.
-	    ufunc->uf_args.ga_len = i;
+	    ufunc->uf_args_visible = arg_idx;
 
 	    arg = ((char_u **)(ufunc->uf_def_args.ga_data))[i];
 	    r = compile_expr0(&arg, &cctx);
 
-	    ufunc->uf_args.ga_len = uf_args_len;
 	    if (r == FAIL)
 		goto erret;
 
@@ -9230,6 +9228,7 @@
 	if (did_set_arg_type)
 	    set_function_type(ufunc);
     }
+    ufunc->uf_args_visible = ufunc->uf_args.ga_len;
 
     /*
      * Loop over all the lines of the function and generate instructions.