A problem about rtx-tiny

When I debug the following program, received an errno:
***errno34: undefined identifier
#include <rtx51tny.h>
#include <reg52.h>
#define INIT 0
#define ONE 1
#define TWO 2
int counter1;
int counter2;

void initial(void) _task_ INIT
{
os_create_task(ONE);
os_create_task(TWO);
os_delete_task(INIT);
}

void job1(void) _task_ ONE
{
while (1)
{
counter1++;
}
}

void job2(void) _task_ TWO
{
while (1)
{
counter2++;
}
}

But when I select "go" from debug menu, I can see the tasklist. That's why? Thanks.
(Keil V6.20)