以下代码在编译时在 代码标记处 出现 probably no zero prefix for hex; or no 'h' suffix; or wrong addressing; or undefined var: [DX] 错误
请问这是为什么
代码的作用是将存在四个字组中的5个学生的成绩分别求总和，使用了双重循环

DATA SEGMENT
COURSE1 
DB 70H,80H,90H,60H,50H
DB 75H,85H,95H,65H,55H
DB 89H,84H,38H,92H,43H
DB 34H,32H,53H,35H,21H
NUM1 DW 5 DUP(?)
DATA ENDS

CODE SEGMENT
ASSUME CS:CODE,DS:DATA

START:
MOV AX,DATA
MOV DS,AX
LEA SI,COURSE1
LEA DI,NUM1
SUB SI,1
MOV CX,5

A1:
INC SI
SUB AX,AX
MOV DX,SI

A2:
MOV BL,4  
ADD AL,[DX]  # <-- 这里报错
DAA     
ADC AH,0
ADD DX,5
DEC BL
JNZ A2
MOV [DI],AX
INC DI
LOOP A1
MOV AH,4CH
INT 21H
CODE ENDS
     END START