Value cannot be null. Parameter name: source on Connector

Hi,

I wrote a function to extend the Connector that relates an account with a contact.

The standard one was not workn

[MappingFunctionCategory(Category = "Custom"), MappingFunction(Description = "Return the string before the comma")]
    public static string ExtractNameBeforeComma(string containingString = "defaultString")
    {
        string result = "";
        string integrationKey = containingString;
        if (integrationKey.Contains(','))
        {
            int index = integrationKey.IndexOf(',');
            result = integrationKey.Substring(0, index);
        }
        return result;
    }

As you can see the code is very simple. Connector creates a value on the Contact\DynamicsIntegrationKey equals to RecID,DynamicsIntegrationKey like this one 53286,bf021a27-8eab-e311-bfb0-0050569239ef

What I need to do is to extract the first number (the recordid) to associate it with the account.

It works fine if the account has a primary contact, but it doesn't work when it does not have it.

The error I can find on the log is: Value cannot be null. Parameter name: source on Connector

Is there any way to avoid this error?