Low-Speed Oscillator on STM32F746

I've got a development board based on the STM32F746 from Mikroelektronika. The board has a 32.768 kHz crystal connected to the LSE oscillator pins. I can't get the oscillator to start and come ready.

Here's my code--any idea what I'm doing wrong?

/*

 * Start the LSE (32 kHZ) oscillator

 */

    RCC->APB1ENR |= RCC_APB1ENR_PWREN;        // enable PWR interface clock

    PWR->CR1 |= PWR_CR1_DBP;                                  // disable backup domain write protection

    RCC->BDCR |= RCC_BDCR_BDRST;                        // reset backup domain

    RCC->BDCR &= ~RCC_BDCR_BDRST;                    // deassert backup domain reset

    RCC->BDCR |= RCC_BDCR_LSEON;                        // enable LSE oscillator

    while(!(RCC->BDCR & RCC_BDCR_LSERDY))

                 ;                                                                             // wait for LSE ready

    RCC->BDCR &= ~(0b11 << 8);                                      //

    RCC->BDCR |= 0b01 << 8;                                            // select LSE as RTC clock

    RCC->BDCR |= RCC_BDCR_RTCEN;                        // enable RTC