Sub always uses key as env prefix

This changes the behavior of Sub to always return a non-nil value with the env key prefix set and only optionally setting a map value as config.

viper.go

@@ -944,20 +944,15 @@ func Sub(key string) *Viper { return v.Sub(key) }
 
 func (v *Viper) Sub(key string) *Viper {
 	subv := New()
-	data := v.Get(key)
-	if data == nil {
-		return nil
-	}
-
-	if reflect.TypeOf(data).Kind() == reflect.Map {
 	subv.parents = append(v.parents, strings.ToLower(key))
 	subv.automaticEnvApplied = v.automaticEnvApplied
 	subv.envPrefix = v.envPrefix
 	subv.envKeyReplacer = v.envKeyReplacer
+	data := v.Get(key)
+	if reflect.TypeOf(data).Kind() == reflect.Map {
 		subv.config = cast.ToStringMap(data)
-		return subv
 	}
-	return nil
+	return subv
 }
 
 // GetString returns the value associated with the key as a string.

viper_test.go

@@ -2619,6 +2619,15 @@ func TestSliceIndexAccess(t *testing.T) {
 	assert.Equal(t, "Static", v.GetString("tv.0.episodes.1.2"))
 }
 
+func TestSubUsesEnvPrefix(t *testing.T) {
+	testutil.Setenv(t, "TEST_SUB_VALUE", "test")
+	v.SetEnvPrefix("TEST")
+	v.AutomaticEnv()
+
+	sub := v.Sub("sub")
+	assert.Equal(t, "test", sub.GetString("value"))
+}
+
 func BenchmarkGetBool(b *testing.B) {
 	key := "BenchmarkGetBool"
 	v = New()