If subpoenaed, you have to appear and testify. You are only precluded from testifying if it would raise some privilege (i.e. 5th Amendment).
You should speak with a lawyer to determine if you have any rights or privilege that can be asserted since the facts didn't state.
Unless there is a court order commanding you to testify you need not do so...but do not ignore You need to write to the person that issued the subpoena explaining why you consider it invalid.
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