confused on this error

void square_manager+<ListInvoices>d__5.MoveNext(): System.NullReferenceException: Object reference not set to an instance of an object.

ListInvoicesResponse result = await square_manager.ListInvoices(); 

public static async Task<ListInvoicesResponse> ListInvoices() {  
        try {
            var result = await client.InvoicesApi.ListInvoicesAsync(locationId: null);
            return result;
        }