程序集分支:简单循环中的程序计数器无效?为什么?

所以我对MIPS程序集不太熟悉;这段代码运行,但它以“invalid program counter”错误结尾,分支如何扰乱程序计数?

main:
    addi $a0, $0, 0xff
    jal func1

func1:
    addi $sp,$sp,-4     
    sw $ra,0($sp)       # save return address
    addi $t1, $0, 3     # if it detects 0xff, loop three times
    beq $a0, 0xff, loop1
    bnez $t1, func_end

loop1:
    subi $t1, $t1, 1
    bnez $t1, loop1
    
func1_end:
    lw $ra, 0($sp)      # restore previous value of 0x($ra)
    addi $sp, $sp, 4    
    jr $ra

li $v0, 10 
syscall