ERROR: Invalid baud rate supplied (when using external mode in Simulink)

Using Matlab 2016a (on Linux) and Simulink with a homegrown external target with a CortexM4 processor.

When interfacing with a serial interface with 921600 and setting MEX-file arguments to: 1 /dev/ttyUSB0 921600 we get:

ERROR: Invalid baud rate supplied (921600)

But using:

>> s=serial('/dev/ttyUSB0','BaudRate',921600);fopen(s);

shows that matlab can set the baudrate to this value as shown with the command "stty -F /dev/ttyUSB0 speed".

How can I set the baudrate to this value in Simulink?

Thanks in advance, Sietse

PS. 115200 as baudrate works fine.