Context free grammar to Chomsky normal form

I have a hard time understanding how to create CNF from following context free grammar CFG:

S -> aSX | B

B -> bBX | ε

X -> a | b

I have a solution

S -> EY | EZ | FY | FZ | ε

E -> YS

F -> ZB

Y -> a

Z -> b

that does not work and can’t figure why, any solutions and explanations would be helpful.