Journey Builder - custom activity stops journey saving

i have a very simple custom activity that is designed to pass some values to a API endpoint for execution. the activity simply launches a window with url and request body as post vars. the post vars are pushed on clickednext event into the inargments along with couple of basic contact attributes email and phone.

no matter with inargments or not, the journey which has the activity in it won’t save with a contact support error msg. as far as i can tell, on save, journey builder passes over a payload which has the right attributes. for some reason, the backend don’t quite like it.

see postmangor and config.json code below.

{
    "metaData": {
        "icon": "images/jb-icon.jpg",
        "iconSmall": "images/jb-icon.jpg"
    },
    "key": "-----",
    "type": "ContactSpace",
    "workflowApiVersion": "1.1",
    "lang": {
        "en-US": {        
            "name": "ContactSpace",
            "description": "Activity simply posts the data to API endpoint."
        }
    },
    "category": "Custom",
    "version": "1.0",
    "edit": {
        "uri": "index.html",
        "maxHeight": 500,
        "maxWidth": 750,
        "minHeight": 300,
        "minWidth": 450,
        "steps": 1
    },
    "userInterfaces": {
        "configModal": {
            "height": 300,
            "width": 300,
            "fullscreen": false
        }
    },
    "arguments": {
        "execute": {
            "inArguments": [{
                "emailAddress": "{{InteractionDefaults.Email}}"
            },
            {
                "phoneNumber": "{{Contact.Attribute.Person.s360a__PhoneNumberMobilePhoneNumber__c}}"
            },
            {
                "call_body": ""
            },
            {
                "call_url" : ""
            },
            {
                "contactIdentifier" : "{{Contact.Key}}"
            }],
            "outArguments": [],
            "timeout": 100000,
            "retryCount": 1,
            "retryDelay": 10000,
            "concurrentRequests" : 5,
            "url": "somedomain/ixn/activities/someact/execute"
        }
    },
    "configurationArguments": {
        "save": {
            "url": "somedomain/ixn/activities/someact/save",
            "body": "",
            "verb": "POST",
            "useJwt": false
        },
        "publish": {
            "url": "somedomain/ixn/activities/someact/publish"
        },
        "validate": {
            "url": "somedomain/ixn/activities/someact/validate"
        }
    },
    "schema": {
        "arguments": {
            "execute": {
                "inArguments": [
                    {
                        "phoneNumber": {
                            "dataType": "Phone",
                            "isNullable": true,
                            "direction": "in"   
                        }
                    },
                    {
                        "emailAddress": {
                            "dataType": "Email",
                            "isNullable": true,
                            "direction": "in"
                        }
                    },
                    {
                        "contactIdentifier": {
                            "dataType": "Text",
                            "isNullable": true,
                            "direction": "in"
                        }
                    },
                    {
                        "call_body": {
                            "dataType": "Text",
                            "isNullable": true,
                            "direction": "in"
                        }
                    },
                    {
                        "call_url": {
                            "dataType": "Text",
                            "isNullable": true,
                            "direction": "in"
                        }
                    }
                ],
                "outArguments": []
            }
        }
    }
}

connection.on('initActivity', function(data) { 
        console.log('initActivity');
        console.log(data);

        if (data) {
            payload = data;
        }

        var hasInArguments = Boolean(
            payload['arguments'] &&
            payload['arguments'].execute &&
            payload['arguments'].execute.inArguments &&
            payload['arguments'].execute.inArguments.length > 0
        );
        var values;

        if (hasInArguments){
            values = payload['arguments'].execute.inArguments.filter(function( obj ) {
                return obj.hasOwnProperty('call_url') || obj.hasOwnProperty('call_body');
            });
        }

        console.log(values);

        if (values && values.length>0){
            $('#call_url').val( values[0].call_url );
            $('#call_body').val( values[1].call_body );
        }
        
        

    });

    connection.on('clickedNext', function(options) {


        console.log('clickedNext');

        var urlvalue = $('#call_url').val();
        var bodyvalue = $('#call_body').val();

        if( !urlvalue ) {
            console.log('empty value');
            // Notify user they need to select a value 
            $('#helloWorldTriggerConfigError').html('<strong style="color: red;">You must enter something</strong>');
            connection.trigger('ready');
        } else {

            payload['arguments'].execute.inArguments = payload['arguments'].execute.inArguments.filter(function( obj ) {
                return !obj.hasOwnProperty('call_url') && !obj.hasOwnProperty('call_body');
            });


            if (!payload.name) payload.name = 'ContactSpace';
            payload['arguments'].execute.inArguments.push({ "call_url": urlvalue });
            payload['arguments'].execute.inArguments.push({ "call_body": bodyvalue  });
            
            payload['metaData'].isConfigured = true;
            connection.trigger('updateActivity', payload);

            
            connection.trigger('nextStep');
            
        }

    });