College Changing Contract Interpretation Retroactively

I teach at a college in Illinois. Faculty are unionized with a faculty contract.

Our faculty contract states: "Bargaining Unit Members will receive an additional $2,000 added to their base salary upon completion of a higher education degree."

I have completed an additional degree, and now the college is claiming it has to be a higher level degree.

However, the written contract does not say this. It specifically says "higher education degree" not "higher level degree". "Higher education" in the United States refers to tertiary education. So my interpretation is that you have to get a degree from a legitimate college, not just a random thing you call a degree, which is reasonable. If "higher education" doesn't refer to "higher education" in the standard meaning, it is hard to tell what it means. They clearly didn't mean "higher (degree in education)" as that would not apply to most people.

It seems to me that allowing one party to interpret the contract differently than it is written largely cancels the purpose of a written and signed employment contract. I don't think they can argue this was a simple scrivener's error, because the college and the union had plenty of opportunity to carefully read and modify the contract before signing it. Maybe they intended to write a higher "level", but...they didn't write that.

So, legal advice of reddit, any advice? How strong is my legal footing?