diff --git a/viper.go b/viper.go
index ca1a2de..cee37b2 100644
--- a/viper.go
+++ b/viper.go
@@ -1267,11 +1267,13 @@ func (v *Viper) MergeConfig(in io.Reader) error {
 }
 
 // MergeConfigMap merges the configuration from the map given with an existing config.
+// Note that the map given may be modified.
 func MergeConfigMap(cfg map[string]interface{}) error { return v.MergeConfigMap(cfg) }
 func (v *Viper) MergeConfigMap(cfg map[string]interface{}) error {
 	if v.config == nil {
 		v.config = make(map[string]interface{})
 	}
+	insensitiviseMap(cfg)
 	mergeMaps(cfg, v.config, nil)
 	return nil
 }
diff --git a/viper_test.go b/viper_test.go
index 94bf5e4..f4263d3 100644
--- a/viper_test.go
+++ b/viper_test.go
@@ -1252,8 +1252,11 @@ func TestMergeConfigMap(t *testing.T) {
 	assert(37890)
 
 	update := map[string]interface{}{
-		"hello": map[string]interface{}{
-			"pop": 1234,
+		"Hello": map[string]interface{}{
+			"Pop": 1234,
+		},
+		"World": map[interface{}]interface{}{
+			"Rock": 345,
 		},
 	}
 
@@ -1261,6 +1264,10 @@ func TestMergeConfigMap(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if rock := v.GetInt("world.rock"); rock != 345 {
+		t.Fatal("Got rock:", rock)
+	}
+
 	assert(1234)
 
 }