malloc causing fault in SoftDevice

I'm using s140 on an nRF52840 and running into a hard fault when trying to malloc 20 bytes. (Yes, I know about memory fragmentation; it's not a concern here.)

app_error_fault_handler give me a counter of 0x1505C and a fault ID of 1 (and info of 0).

The sdh has been enabled, but bluetooth is not enabled.

Given that I can find no info on what the Softdevice is attempting to assert, I'm at a bit of a loss.