Supplier Site REST API Returns No Data

Summary:

I am using the GET RESt API for supplier sites and no data is being returned. All of the other supplier APIs are returning data with the exception of sites.

Content (please ensure you mask any confidential information):

I am using: /fscmRestApi/resources/11.13.18.05/suppliers/{SupplierId}/child/sites

I tried: /fscmRestApi/resources/11.13.18.05/suppliers?q=SupplierNumber IN('<SupplierNumber>')&expand=sites.assignments&fields=SupplierId,SupplierNumber,AlternateName;sites:SupplierSiteId,SupplierSite;sites.assignments:AssignmentId,ChartOfAccountsId&onlyData=true as recommended in Doc ID 2780914.1 but it also returns no results

Is there something else I need to change in the above URL or could this be because of another issue?