Is Post Correspondence Problem in NP?

I just read some pages in Sipser's book Introduction to Theory of Computation about Post Correspondence Problem, and I'm thinking that PCP is actually in NP. The certifier is: for an input configuration of pile $$(t_1/b_1, t_2/b_2,...t_n/b_n)$$ concatenating $t_1, t_2,...,t_n$ as a string $t$ and concatenating $b_1, b_2, ..., b_n$ as a string $b$, then compare $t$ and $b$ to see if the two are equal and then conclude that the input is actually a solution to PCP.