What is the test for "negation of mens rea" for driving while disqualified?

In appropriate circumstances, therefore, the court may conclude that a "negation of mens rea" has occurred. In this regard, in R v. Prue and Baril 1979 CanLII 227 (SCC), [1979] 46 CCC (2d) 257 (S.C.C.), the two accuseds had been convicted of impaired driving under the then existing s. 236 of the Criminal Code. As well, their operators' licences were automatically suspended in accord with the provincial vehicle administration statute which was in force at the time. However, on the facts, they were not notified by the authorities of the disqualifications, and neither was aware of it. In due course they were both charged with drive while disqualified under the Criminal Code.