"libertas arbitrium"

Dear all,
I am wondering if this locution is grammatically valid. I foun it in the following passage:
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The Latin “gratia” is related to “gratuitus” which means “not paid for.” Consequently, Latin theology literally regards grace in financial terms. (Modern banking still refers to a “grace period.”) Understandably, the final judgement is a reckoning of accounts. The issue of free will, “libertas arbitrium,” is likewise understood in this context. Libertas is freedom, independence. Arbitrium means arbiter in a legal sense. It can also mean financial expenses, that which is due after arbitration. Free will is the internal exercise of judgment, decision making.
http://cte.rockhurst.edu/facultydetail.aspx?pgID=1295&newsID=56&exCompID=1622
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Before that, I had encountered only the locution “liberum arbitrium”, the grammar of which is clear to me (an adjective and a noun, both neuter nominative). But “libertas arbitrium” appear to me as just two nouns in the nominative. Is this right? And if so, what sense can it possibly make as a locution?
Many thanks in advance.
Arkadi

it seems to be an error for “libertatis arbitrium”

Apparently, you are right. I made a search for “libertatis arbitrium” and found it in Boethius.