error A2070:invalid instruction operands

aveL:
mov ax,word ptr [stu+si] ;判断学号空间是否为空，若为空则跳出
cmp ax,0
jz over_ave
mov ax,0
mov dx,0
mov dl,[stu+si]
mov al,[stu+si+1]
sub dl,30h
sub al,30h
shl dl,4
add dx,ax
add cx,dx
add si,12
inc bx
jmp aveL
在SHL DL,4处提示error A2070:invalid instruction operands，求解决办法

参考：http://blog.163.com/bestfighter_210@126/blog/static/1036188720120314317932/