Error when doing PB8.into_alternate_open_drain

[dbrgn]

I'm trying to put PB8 into alternate open drain mode, so I can use it for I2C.

37          // Get access to PORTB pins
38          let mut gpiob = dp.GPIOB.split(&mut rcc.apb2);
39
40          // We want to use pin PB8 as SCL1 and PB9 as SDA1 
41          let scl1 = gpiob.pb8.into_alternate_open_drain(&mut gpiob.crh);
42          let sda1 = gpiob.pb9.into_alternate_open_drain(&mut gpiob.crh);

When I run this in GDB, something crashes and the asm::bkpt() default handler is called.

Program received signal SIGTRAP, Trace/breakpoint trap.
cortex_m_rt::default_handler (ef=0x20004d80) at /home/danilo/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-rt-0.3.13/src/lib.rs:664
664         asm::bkpt();
(gdb) bt
#0  cortex_m_rt::default_handler (ef=0x20004d80) at /home/danilo/.cargo/registry/src/github.com-1ecc6299db9ec823/cortex-m-rt-0.3.13/src/lib.rs:664
#1  <signal handler called>
#2  0x08001830 in <stm32f103xx_hal::gpio::gpiob::PB8<MODE>>::into_alternate_open_drain (self=..., cr=0x20004e78)
    at /home/danilo/.cargo/git/checkouts/stm32f103xx-hal-7d0db38b85df9102/80b2099/src/gpio.rs:191
#3  0x0800249e in bluepill_sgp30::main () at src/main.rs:41

Any idea what the reason for this could be? I'm still quite new to microcontrollers with the complexity of a Cortex M3 :)