Form IMM 5406, Additional Family Information, requires you to either identify your current spouse, or certify that you have no current or former spouse or common-law partner. So if you do not have a current spouse, YES, you must identify your former common-law partner.
After taking a look at this form, you'll probably wonder where or how, since section A appears to be for current spouses/partners, not former, but then you can't sign the declaration either. In my case, I wrote in "see attached explanation" instead of signing the declaration, and then put my ex-spouse's information on a separate sheet and wrote (and signed) a statement that I did not currently have a spouse...