Catalysis of the reaction: cis-2-chloro-4-carboxymethylenebut-2-en-1,4-olide = trans-2-chloro-4-carboxymethylenebut-2-en-1,4-olide.
2-chloro-4-carboxymethylenebut-2-en-1,4-olide cis-trans-isomerase activity