My unpaid internship VS my paid part-time job?

So I am currently splitting time between an unpaid internship and a paid part time job.

Tomorrow is a a very busy day (its unusual) for the internship. They asked me if i could take off from my job (tomorrow) to help them.

At my other job, we are behind on a project and people are on vacation so there are less people around to work. And they PAY me, so obviously I am not going to lose a whole day's worth of pay for that.

I have been at the internship longer than my part time job.

I told my boss at the internship I could not take tomorrow off, and she hasn't responded to my voice mail.

Is it wrong of them for me to request off from a paid job to help out on an unpaid job?

Will be internship boss be angry at me for not being able to come?

I love both companies. But I have certain days I commit to each company. Days at my paid job are not flexible like my internship is.