Can a Catholic serve in a non-denominational church?

I am an instrumental musician (brass instrument) who would like to serve in my parish music ministry, but was informed that there was no place for me, except for the possibility of “special occasions.” (That hasn’t happened). A local non-denominational church invited me (by way of mutual acquaintances) to serve with their music ministry 1-2 times per month. I have been doing that since February. Is that wrong?

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*]May I serve in a non-Catholic music ministry?
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