fmgr_info_cxt_security() modifies variable?

Hi, I'm calling an arbitrary user-defined function from the backend.
Although I can do it via FunctionCallInvoke, I have a weird problem when
calling fmgr_info. The call results in a argument variable (eventType)
being cleared. A gdb hardware watch says that the variable is modified by
this line (185) in fmgr.c: finfo->fn_extra = NULL; Any ideas of what is
happening?

For simplicity, in the following snippet I have hardcoded foid to some
(PGPSQL) function id and removed the rest of the call-related statements.

static void execEventTypeFunc(char *eventType, Oid funcId)
{
FmgrInfo *finfo;
FunctionCallInfoData fcinfo;
Datum result;
Oid foid = 17283;

finfo = (FmgrInfo *) palloc0(sizeof(FmgrInfo));

fmgr_info(foid, finfo);

...
}

Thanks,

Luis