AngularJs CheckBox conundrum

My view has

<input type="checkbox" class="check_box" ng-model="campaign.paused"  
       ng-click="CampaignPauseClicked(campaign, $event)" />

<p>campaign.paused == </p>                           

with the <p> being for debugging. It shows false, as it shoudl, given the data, but in the controller

$scope.CampaignPauseClicked = function(campaign, $event)
{
    campaign.paused = ! campaign.paused;

when I breakpoint on the first code line, the value of campaign.paused is true (!).

I have searched the code and campaign.paused is not being written elsewhere.

Any idea what could be happening here?