was or were

Was. Series is both a singular and a plural form. When it has the singular sense of "one set," it takes a singular verb, even when series is followed by of and a plural noun: A series of lectures is scheduled. When it has the plural sense of "two or more sets," it takes a plural verb: Two series of lectures are scheduled: one for experts and one for laypeople.

Thanks, indeed.

Yea thanks Cen, on behalf of the Cheery Ignoramus (me)