diff --git a/build.go b/build.go
index 0b3e4ab..f3838a7 100644
--- a/build.go
+++ b/build.go
@@ -167,14 +167,13 @@ MAIN:
 	}
 
 	// Scan through argument positionals to ensure optional is never before a required.
-	last := true
-	for i, p := range node.Positional {
-		if !last && p.Required {
-			return nil, fmt.Errorf("argument %q can not be required after an optional", p.Name)
+	var last *Value
+	for i, curr := range node.Positional {
+		if last != nil && !last.Required && curr.Required {
+			return nil, fmt.Errorf("%s: required %q can not come after optional %q", node.FullPath(), curr.Name, last.Name)
 		}
-
-		last = p.Required
-		p.Position = i
+		last = curr
+		curr.Position = i
 	}
 
 	return node, nil
diff --git a/kong_test.go b/kong_test.go
index 4c9d5fb..f0844ba 100644
--- a/kong_test.go
+++ b/kong_test.go
@@ -1382,3 +1382,19 @@ func TestOptionReturnsErr(t *testing.T) {
 	require.Error(t, err)
 	require.Equal(t, "option returned err", err.Error())
 }
+
+func TestEnumValidation(t *testing.T) {
+	var cli struct {
+		Enum string `arg:"" enum:"one,two"`
+	}
+	_, err := kong.New(&cli)
+	require.Error(t, err)
+}
+
+func TestEnumValidationSlice(t *testing.T) {
+	var cli struct {
+		Enum []string `arg:"" enum:"one,two"`
+	}
+	_, err := kong.New(&cli)
+	require.NoError(t, err)
+}
diff --git a/tag.go b/tag.go
index d76c418..a817c88 100644
--- a/tag.go
+++ b/tag.go
@@ -138,7 +138,7 @@ func parseTagString(s string) (*Tag, error) {
 	t := &Tag{
 		items: items,
 	}
-	err = hydrateTag(t, "", false)
+	err = hydrateTag(t, nil)
 	if err != nil {
 		return nil, fmt.Errorf("%s: %s", s, err)
 	}
@@ -158,14 +158,20 @@ func parseTag(parent reflect.Value, ft reflect.StructField) (*Tag, error) {
 	t := &Tag{
 		items: items,
 	}
-	err = hydrateTag(t, ft.Type.Name(), ft.Type.Kind() == reflect.Bool)
+	err = hydrateTag(t, ft.Type)
 	if err != nil {
 		return nil, failField(parent, ft, "%s", err)
 	}
 	return t, nil
 }
 
-func hydrateTag(t *Tag, typeName string, isBool bool) error {
+func hydrateTag(t *Tag, typ reflect.Type) error { // nolint: gocyclo
+	var typeName string
+	var isBool bool
+	if typ != nil {
+		typeName = typ.Name()
+		isBool = typ.Kind() == reflect.Bool
+	}
 	var err error
 	t.Cmd = t.Has("cmd")
 	t.Arg = t.Has("arg")
@@ -220,7 +226,8 @@ func hydrateTag(t *Tag, typeName string, isBool bool) error {
 	}
 	t.PlaceHolder = t.Get("placeholder")
 	t.Enum = t.Get("enum")
-	if t.Enum != "" && !(t.Required || t.Default != "") {
+	scalarType := (typ == nil || !(typ.Kind() == reflect.Slice || typ.Kind() == reflect.Map))
+	if t.Enum != "" && !(t.Required || t.Default != "") && scalarType {
 		return fmt.Errorf("enum value is only valid if it is either required or has a valid default value")
 	}
 	passthrough := t.Has("passthrough")