When is the sub-bicategory $[B, C]$ of $\textbf{Lax}(B, C)$ a strict 2-category?

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Let $B, C$ be bicategories and let $\textbf{Lax}(B, C)$ denote the functor bicategory. Denote by $[B, C]$ the sub-bicategory consisting of pseudofunctors, pseudonatural transformations, and modifications. I've read in several places that $[B, C]$ is a strict 2-category if $C$ is, but haven't managed to find a proof. Could someone point me in the right direction?