A prefix of the character sequence you were trying to bind was already bound. Usually, the sequence is "ESC [", in which case you should evaluate this form first:
(define-key esc-map "[" nil)
NOTE: By default, "ESC [" is bound to backward-paragraph, and if you do this you will lose this key binding. For most people, this is not a problem.
See section How do I bind keys (including function keys) to commands?.