vTaskDelete passing NULL

I am using the EFM32gg M4. strange thing when I pass NULL to vTaskDelete I get an assert and I cant figure out why.

vTaskDelete passing NULL

Even if i try this still causing an assert static void SYSTEMinitTask(void *pvParameters) { for (;;) { SYSTEMinit();
     if(xTaskSysTaskInit != NULL)
     {
         vTaskDelete(xTaskSysTaskInit);
     }
}
vTaskDelete(NULL);
}

vTaskDelete passing NULL

Which assert() is being hit? Can you step through in the debugger to see why?