Material variation rule does not apply where contracted out

Fifth Third Bank v. O'Brien, 2013 ONCA 5 holds the material variation rule vacating a guarantee does not apply where the contracts supporting the guarantee contracts out of the rule.  The rule provides if the principle obligation is varied that, absent contract, voids the guarantee:

[17]       Finally, the motion judge held that the material variation rule discussed inHolmes v. Brunskill, [1987] 3 Q.B.D. 495 and Manulife Bank of Canada v. Conlin, [1996] 3 S.C.R. 415 does not apply here as the changes made to MPI's loan facilities were authorized by the principal loan agreements and guarantees.  Having reviewed the agreements in question, we see no basis on which to interfere with this holding.