diff --git a/src/app/units/states/tasks/definition/definition.coffee b/src/app/units/states/tasks/definition/definition.coffee
index bd0bad8241..108f82068f 100644
--- a/src/app/units/states/tasks/definition/definition.coffee
+++ b/src/app/units/states/tasks/definition/definition.coffee
@@ -1,5 +1,4 @@
-angular.module('doubtfire.units.states.tasks.definition', [
-])
+angular.module('doubtfire.units.states.tasks.definition', [])
 
 #
 # Mark tasks by task definition ID
@@ -8,23 +7,45 @@ angular.module('doubtfire.units.states.tasks.definition', [
   $stateProvider.state 'units/tasks/definition', {
     parent: 'units/tasks'
     url: '/definition/{taskKey:any}'
-    # We can recycle the task inbox, switching the data source scope variable
+
+    # Reuse the inbox UI directly (controller handles definition-specific behaviour)
     templateUrl: "units/states/tasks/inbox/inbox.tpl.html"
     controller: "TaskDefinitionStateCtrl"
+
     params:
       taskKey: dynamic: true
+
     data:
       task: "Task Explorer"
       pageTitle: "_Home_"
       roleWhitelist: ['Tutor', 'Convenor', 'Admin', 'Auditor']
-   }
+  }
 )
 
 .controller('TaskDefinitionStateCtrl', ($scope, newTaskService) ->
+
+  # Ensure taskData exists (prevents "Cannot set property 'source' of undefined")
+  $scope.taskData ?= {}
+
+  # Swap the data source to definition-based query, while reusing inbox UI
   $scope.taskData.source = newTaskService.queryTasksForTaskExplorer.bind(newTaskService)
   $scope.taskData.taskDefMode = true
+
+  # Show inbox search UI options on the Task Explorer view
   $scope.showSearchOptions = true
-  $scope.filters = {
-    taskDefinitionIdSelected: _.first($scope.unit.taskDefinitions)?.id
-  }
+
+  # Ensure filters exists (safe when navigating directly before unit loads)
+  $scope.filters ?= {}
+
+  setDefaultDefId = (defs) ->
+    return if $scope.filters.taskDefinitionIdSelected?
+    return unless defs? and defs.length > 0
+    $scope.filters.taskDefinitionIdSelected = defs[0].id
+
+  # If already loaded
+  setDefaultDefId($scope.unit?.taskDefinitions)
+
+  # If loaded later
+  $scope.$watch 'unit.taskDefinitions', (defs) ->
+    setDefaultDefId(defs)
 )