Unable to pass complex object parameters from Lightning client controller to Apex server controller

So this code works as gateway is a simple string. However whenever I try to make gateway an object I start getting errors

       /*******************************************************************************************************
        * @description Test saving configuration
        */
        var saveConfiguration = component.get("c.testSomething");
        // Set save configuration parameters.
        saveConfiguration.setParams({
            testSome : 'Hello World!',
            config : {
                gateway: 'Stripe'
            }
        });
        // Setup save configuration callback
        saveConfiguration.setCallback(this, function(a) {
                if (a.getState() === "SUCCESS") {
                   // Do something with return value
                } else if (a.getState() === "ERROR") {
                    $A.error("Errors", a.getError());
                    $A.log("Errors", a.getError());
                }
            });
        // Queue the transaction action
        $A.enqueueAction(saveConfiguration);

This code gives me an error when trying to access the parameter map in the server side controller. 

       /*******************************************************************************************************
        * @description Test saving configuration
        */
        var saveConfiguration = component.get("c.testSomething");
        // Set save configuration parameters.
        saveConfiguration.setParams({
            testSome : 'Hello World!',
            config : {
                gateway: 'Stripe'
            }
        });
        // Setup save configuration callback
        saveConfiguration.setCallback(this, function(a) {
                if (a.getState() === "SUCCESS") {
                   // Do something with return value
                } else if (a.getState() === "ERROR") {
                    $A.error("Errors", a.getError());
                    $A.log("Errors", a.getError());
                }
            });
        // Queue the transaction action
        $A.enqueueAction(saveConfiguration);

Here's my server side controller method:

@AuraEnabled
public static void testSomething(String testSome, Map<String, Object> config) {
    System.debug(config.get('gateway'));
}