Error A2070

[=?Utf-8?B?TWljaGFlbA==?=]

my simple16-bit program has the following line:

sub [BX], 30h

Clearly, I'm trying to convert an ASCII number to a single-digit hex number.

The error message I'm getting (A2070) refers to "invalid instruction
operands."

So what am I doing wrong?


Thanks,

Michael