helper/schema: TypeMap computed should mark diff as cmputed

helper/schema/schema.go

@@ -584,6 +584,13 @@ func (m schemaMap) diffMap(
 
 	// If the new map is nil and we're computed, then ignore it.
 	if n == nil && schema.Computed {
+		// If we don't have an old value, this whole map is computed
+		if o == nil {
+			diff.Attributes[k] = &terraform.ResourceAttrDiff{
+				NewComputed: true,
+			}
+		}
+
 		return nil
 	}
 

helper/schema/schema_test.go

@@ -1292,6 +1292,29 @@ func TestSchemaMap_Diff(t *testing.T) {
 			Err: false,
 		},
 
+		{
+			Schema: map[string]*Schema{
+				"vars": &Schema{
+					Type:     TypeMap,
+					Computed: true,
+				},
+			},
+
+			State: nil,
+
+			Config: nil,
+
+			Diff: &terraform.InstanceDiff{
+				Attributes: map[string]*terraform.ResourceAttrDiff{
+					"vars": &terraform.ResourceAttrDiff{
+						NewComputed: true,
+					},
+				},
+			},
+
+			Err: false,
+		},
+
 		{
 			Schema: map[string]*Schema{
 				"config_vars": &Schema{