diff --git a/src/Illuminate/Database/Eloquent/Relations/MorphTo.php b/src/Illuminate/Database/Eloquent/Relations/MorphTo.php
index 2f449876fac7..8a6c7a2cbca8 100644
--- a/src/Illuminate/Database/Eloquent/Relations/MorphTo.php
+++ b/src/Illuminate/Database/Eloquent/Relations/MorphTo.php
@@ -226,7 +226,7 @@ protected function matchToMorphParents($type, Collection $results)
     public function associate($model)
     {
         $this->parent->setAttribute(
-            $this->foreignKey, $model instanceof Model ? $model->getKey() : null
+            $this->foreignKey, $model instanceof Model ? $model->{$this->ownerKey ?: $model->getKeyName()} : null
         );
 
         $this->parent->setAttribute(
diff --git a/tests/Database/DatabaseEloquentMorphToTest.php b/tests/Database/DatabaseEloquentMorphToTest.php
index a6bd248ff563..0b0741de406c 100644
--- a/tests/Database/DatabaseEloquentMorphToTest.php
+++ b/tests/Database/DatabaseEloquentMorphToTest.php
@@ -132,7 +132,7 @@ public function testAssociateMethodSetsForeignKeyAndTypeOnModel()
         $relation = $this->getRelationAssociate($parent);
 
         $associate = m::mock(Model::class);
-        $associate->shouldReceive('getKey')->once()->andReturn(1);
+        $associate->shouldReceive('getAttribute')->once()->andReturn(1);
         $associate->shouldReceive('getMorphClass')->once()->andReturn('Model');
 
         $parent->shouldReceive('setAttribute')->once()->with('foreign_key', 1);