SVC calls causes an exception

Hello Cortex M3 experts,

I have a FreeRTOS task that runs in Thread/unprivileged mode. When that task tries to execute a SVC instruction, I get a hard fault. But if the task is launched in privileged mode, that does not happen. The strange thing is that it happens in this code:

__asm portBASE_TYPE portRaisePrivilege( void )
{
        mrs r0, control
        tst r0, #1                                                      ; Is the task running privileged? */
        itte ne
        movne r0, #0                                            ; CONTROL[0]!=0, return false. */
        svcne #portSVC_RAISE_PRIVILEGE          ; Switch to privileged. */
        moveq r0, #1                                            ; CONTROL[0]==0, return true. */
        bx lr
}

which is invoked by these calls:

  void vPortExitCritical( void )
{
        portBASE_TYPE xRunningPrivileged = portRaisePrivilege() ;

        uxCriticalNesting--;
        if( uxCriticalNesting == 0 )
        {
                portENABLE_INTERRUPTS();
        }

    portRESET_PRIVILEGE( xRunningPrivileged ) ;
}

and

void vPortEnterCritical( void )
{
    portBASE_TYPE xRunningPrivileged = portRaisePrivilege() ;

        portDISABLE_INTERRUPTS();
        uxCriticalNesting++;

    portRESET_PRIVILEGE( xRunningPrivileged ) ;
}

If I call the functions like this:

vPortEnterCritical() ;
vPortExitCritical() ;

the hard fault will occur during the call to "vPortExitCritical".
Any ideas?