warning: #1441-D: nonstandard cast on lvalue

Hello guys, i am having this errore with the following code:

startLinst(Timer_u *head){ Timer_u *ptr;

(Timer_u *)ptr->list1->pnext = *head;
} where head is the pointer to the head of a linked list.

the ADT is:
typedef union TIMER_U{ Timer4 *timer4; Timer3 *timer3; Timer2 *timer2; Timer1 *timer1;
}Timer_u;

typedef struct TIMER1{ unsigned char TimerID; unsigned char cnt; unsigned char processo:6; unsigned char type:2; union Timer_u *pnext;
}Timer1;

Can anyone help me to avoid this warning message?