Simulink Startfcn Callback fails when using serial() function

Matlab 2014a. I have this following function:

function s = open_serial_port()
s = serial('COM2');
set(s,'BaudRate',115200,'Databits',8,'Parity','none','Stopbit',1,'Terminator','LF');
fopen(s);
fprintf(s,'*IDN?')
out = fscanf(s);
disp(out);

Calling it from the Command Window works fine. Calling it from Simulink Startfcn callback does not work, with the following error:

Open failed: Port: COM2 is not available. Available ports: COM1.
Use INSTRFIND to determine if other instrument objects are connected to the requested device.

For some reason COM2 is available from Command Window but not from Simulink Callback?