diff --git a/vlib/v/parser/fn.v b/vlib/v/parser/fn.v
index d35c29a906..8317f1ade6 100644
--- a/vlib/v/parser/fn.v
+++ b/vlib/v/parser/fn.v
@@ -158,6 +158,9 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 	args2, is_variadic := p.fn_args()
 	args << args2
 	for i, arg in args {
+		if p.scope.known_var(arg.name) {
+			p.error('redefinition of parameter `$arg.name`')
+		}
 		p.scope.register(arg.name, ast.Var{
 			name: arg.name
 			typ: arg.typ
diff --git a/vlib/v/tests/nested_map_test.v b/vlib/v/tests/nested_map_test.v
index 92f3a1f282..c193cd6d28 100644
--- a/vlib/v/tests/nested_map_test.v
+++ b/vlib/v/tests/nested_map_test.v
@@ -9,7 +9,7 @@ fn test_nested_maps() {
 	assert x["a"]["b"] == 5
 	x["a"]["b"] = 7
 	assert x["a"]["b"] == 7
-	y := map[string]map[string]map[string]int
+	mut y := map[string]map[string]map[string]int
 	y["a"] = map[string]map[string]int
 	y["a"]["b"] = map[string]int
 	assert y["a"]["b"]["c"] == 0