SIM900 and GPIO5 trouble

Hello. I've got the following trouble on a SIM900 board. I can use GPIO1 to GPIO4 and GPIO6 to GPIO8 as output, but any attempt on GPIO5 does nothing.

The AT command I send is : AT+SGPIO=0,n,1,1 where n is the GPIO number. It works with any number from 1 to 8 except for number 5. The answer given by the SIM900 is always OK. No warning.

Does anybody as an idea of what could be wrong ?

Thanks for your help.