Arduino Zero - Using SERCOM5 Primary Pads [0] and [1]

Hi there,
I modified a Zero board (i.e. using a Zero core variant with the SAMD21 chip).
I wanted to use SERCOM5 pads [0] and [1] but the pins are not all available in "variant.cpp."

The two allowed pins on the SAMD21 are PB02 and PB03, one of which is designated analog pin (pin number 19) and one is the RX LED pin (pin 25).

I tried re-assigning these pins at compile time to:

{ PORTB,  3, PIO_SERCOM, PIN_ATTR_DIGITAL, No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_NONE }, //pin 25

and

{ PORTB,  2, PIO_SERCOM, PIN_ATTR_DIGITAL, No_ADC_Channel, NOT_ON_PWM, NOT_ON_TIMER, EXTERNAL_INT_2 }, //pin 19

I also removed the Uart Serial command and SERCOM5_Handler() from variant.cpp.

Then I declare my TwoWire interface:

 TwoWire tw5(&sercom5, 19, 25);

and include the following code in my setup:

    tw5.begin(); //pressure sensor 1
    pinPeripheral(19, PIO_SERCOM); //change functionality of PIO, SDA1, sercom5
    pinPeripheral(25, PIO_SERCOM); //SCL1, sercom5

I have used this approach for other Sercom connections, i.e. sercom3 (pins 20, 21).

However, when I call the Two-wire command to receive something from a device, the code hangs (it does not do this for the other Two-wire operations on other Sercoms).

tw5.requestFrom(ADDR,4,1); //get 4 bytes of data

Am I still missing a declaration that is causing Sercom5 to not respond? Perhaps TXC or RXC do not clear properly?

Thanks in advance for any advice!