myav
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1. He probably wrote this right after ZVSE.
2. v300 or undefined at the time of access. Or it goes outside the 0..43 range.
I'll probably explain it more clearly.
1. The !!BM command can only be used inside: !?FU, !?BR, !?BG, !?BF. Well, maybe I forgot something else. In other cases, it will throw an error.
2. If, at the time of accessing !!BMv300, our variable v300 does not have one of the values: 0..43, it will again throw an error.
There may be other options, but it is unlikely that they are the cause.