Mailbox_post() fails even when numPendingMsgs = 0

Hi,

I'm using a mailbox with 1 message:

    Mailbox_Params mbox_params;
    Mailbox_Params_init(&mbox_params);
    Error_init(&eb);
    Mailbox_Handle h = Mailbox_create(4, 1, &mbox_params, &eb);
    if (Error_check(&eb))
        System_abort("hMailbox create failed");

I call the mailbox pend from a task with BIOS_WAIT_FOREVER:

while (Mailbox_pend(h, &val, BIOS_WAIT_FOREVER)) {}

I call the mailbox post from a clock swi with BIOS_NO_WAIT:

Bool r = Mailbox_post(h, &q, BIOS_NO_WAIT);
if (r == FALSE)
{
    // here both Mailbox_getNumFreeMsgs()=0 and Mailbox_getNumPendingMsgs()=0
}

When my Mailbox_post() fails, I assume it's because of a timeout if the mailbox is full. However, if I print out the number of messages ready to be read in the mailbox, I get 0. Shouldn't this be 1? Why does the Mailbox_post() fail if there are no messages in the mailbox ?

Any insight on this matter would be helpful. Thanks !