diff --git a/fmt/format.h b/fmt/format.h
index 474cdf4e..70564e19 100644
--- a/fmt/format.h
+++ b/fmt/format.h
@@ -1514,7 +1514,8 @@ class ArgVisitor {
    */
   Result visit(const Arg &arg) {
     switch (arg.type) {
-    default:
+    case Arg::NONE:
+    case Arg::NAMED_ARG:
       FMT_ASSERT(false, "invalid argument type");
       return Result();
     case Arg::INT:
diff --git a/test/util-test.cc b/test/util-test.cc
index 838b3b14..1bb7e5d1 100644
--- a/test/util-test.cc
+++ b/test/util-test.cc
@@ -708,7 +708,7 @@ TEST(ArgVisitorTest, VisitUnhandledArg) {
 
 TEST(ArgVisitorTest, VisitInvalidArg) {
   Arg arg = Arg();
-  arg.type = static_cast<Arg::Type>(Arg::CUSTOM + 1);
+  arg.type = static_cast<Arg::Type>(Arg::NONE);
   EXPECT_ASSERT(TestVisitor().visit(arg), "invalid argument type");
 }