grammatical number - Is "an ocean of flowers" singular or plural?

'There are an ocean of flowers.' I saw this sentence in the Internet. Is it grammatically correct? I was wondering whether 'an ocean of flowers' should be treated as singular or plural.

If it is singular, then the verb should be 'is' - There is an ocean of flowers.

Apparently, the writer consider 'an ocean of flowers' a plural subject, as the verb 'are' was used. But should it be plural when the phrase starts with 'an' ('AN ocean of...')?

Thanks!