Merge pull request #1315 from hashicorp/b-heroku-config-vars-2

providers/heroku: empty config vars block shouldn't panic [GH-1211]

builtin/providers/heroku/resource_heroku_app.go

@@ -358,16 +358,20 @@ func updateConfigVars(
 	vars := make(map[string]*string)
 
 	for _, v := range o {
+		if v != nil {
 			for k, _ := range v.(map[string]interface{}) {
 				vars[k] = nil
 			}
 		}
+	}
 	for _, v := range n {
+		if v != nil {
 			for k, v := range v.(map[string]interface{}) {
 				val := v.(string)
 				vars[k] = &val
 			}
 		}
+	}
 
 	log.Printf("[INFO] Updating config vars: *%#v", vars)
 	if _, err := client.ConfigVarUpdate(id, vars); err != nil {