addDynalink method causing "Wrong argument types for comparison."

So I wanted a way to open the CustInvoiceJournal form and only show invoices that are tagged with a ProjectId that corresponds to the selected Project on the parent form. In other words, duplicate the functionality that you get from Inquiries - Tax Invoice button on the Sales Table Form but look for a ProjectId instead of an SalesId.

Buried inside the CustInvoiceJour Data Source init method I found the code it uses

if (element.args().dataset() == tablenum(SalesTable))
    {
        this.query().dataSourceTable(tablenum(CustInvoiceJour)).addRange(fieldnum(CustInvoiceJour, RefNum)).value(queryValue(RefNum::SalesOrder));
        this.query().dataSourceTable(tablenum(CustInvoiceJour)).clearDynalinks();

        queryDataSourceLink = this.query().dataSourceTable(tablenum(CustInvoiceJour)).addDataSource(tablenum(CustInvoiceSalesLink));
        queryDataSourceLink.relations(true);
        queryDataSourceLink.addDynalink(fieldnum(CustInvoiceSalesLink, OrigSalesId),
                                        element.args().record(),
                                        fieldnum(SalesTable, SalesId));
    }

So I added in a bit after it

if (element.args().dataset() == tablenum(HIL_ProjectTable) && element.args().caller().name() == formstr(HIL_ProjectTable))
    {
        this.query().dataSourceTable(tablenum(CustInvoiceJour)).clearDynalinks();

        queryDataSourceLink = this.query().dataSourceTable(tablenum(CustInvoiceJour)).addDataSource(tablenum(HIL_ProjectTable));
        queryDataSourceLink.relations(true);
        queryDataSourceLink.addDynalink(fieldnum(CustInvoiceJour, HIL_ProjectId),
                                        element.args().record(),
                                        fieldnum(HIL_ProjectTable, ProjectId));

    }

But when I click on my Inquiries - Tax Invoice button in my Projects table I get this error message in the log 

Error executing code: Wrong argument types for comparison.

(C)\Classes\QueryRun\next

(C)\Classes\FormDataSource\executeQuery
(C)\Classes\FormDataSource\linkActive

If I remove the queryDataSourceLink.addDynalink command it opens without the error message (but displays all invoices with a valid HIL_ProjectId)
so it feels like my addDynalink command is wrong... but for the life of me I can't work out how.

I added in a breakpoint and checked the element.args().record() that's being fed to it, but it seems fine and shows a valid projectId. 

Both the HIL_ProjectId in the CustInvoiceJour Table and the ProjectId in the HIL_ProjectTable are the same HIL_Project Extended Data Type.
I've added a relation to the data dictionary on my HIL_Project Table linking to the CustInvoiceJour...

I'll try a full recompile tonight but I don't think I've ever had issues with Forms & Tables before. When classes are involved it seems to get more complicated.

Any ideas?