I know it works differently on the other side of the pond, but in BrE, when the subject consists of a unit noun (the head of a noun phrase which realizes the subject) followed by an 'of' prepositional complement, the verb in grammatical concord agrees in number with the head. In the sentence in question, "of" is optional, and with or without "of" the meaning is the same.
'There were a (couple)(few) passages I found difficult to understand.'
'I have a (couple)(few) things I have to to do before I can go.'
'Couples' is the plural form of 'couple'. only when "couple" means 'a pair', as in a man and a woman.
'couple is singular and so has to be the verb. No, in the sentence in question "couple" means 'two' or a 'few', which is clearly plural. (more than one)
At least that's the view over here.