diff --git a/tests/src/gui/testqgsfeaturelistcombobox.cpp b/tests/src/gui/testqgsfeaturelistcombobox.cpp
index 0ce584ef56d..9bd19c2ea53 100644
--- a/tests/src/gui/testqgsfeaturelistcombobox.cpp
+++ b/tests/src/gui/testqgsfeaturelistcombobox.cpp
@@ -80,6 +80,7 @@ void TestQgsFeatureListComboBox::init()
 {
   // create layer
   mLayer.reset( new QgsVectorLayer( QStringLiteral( "LineString?field=pk:int&field=material:string&field=diameter:int&field=raccord:string" ), QStringLiteral( "vl2" ), QStringLiteral( "memory" ) ) );
+  mLayer->setDisplayExpression( QStringLiteral( "pk" ) );
 
   // add features
   mLayer->startEditing();
diff --git a/tests/src/gui/testqgsrelationreferencewidget.cpp b/tests/src/gui/testqgsrelationreferencewidget.cpp
index 8432ac369f4..846cccfa734 100644
--- a/tests/src/gui/testqgsrelationreferencewidget.cpp
+++ b/tests/src/gui/testqgsrelationreferencewidget.cpp
@@ -100,6 +100,7 @@ void TestQgsRelationReferenceWidget::init()
   QgsProject::instance()->addMapLayer( mLayer1.get(), false, false );
 
   mLayer2.reset( new QgsVectorLayer( QStringLiteral( "LineString?field=pk:int&field=material:string&field=diameter:int&field=raccord:string" ), QStringLiteral( "vl2" ), QStringLiteral( "memory" ) ) );
+  mLayer2->setDisplayExpression( QStringLiteral( "pk" ) );
   QgsProject::instance()->addMapLayer( mLayer2.get(), false, false );
 
   // create relation
diff --git a/tests/src/python/test_qgsfieldformatters.py b/tests/src/python/test_qgsfieldformatters.py
index 214d8596664..fb8e179f60e 100644
--- a/tests/src/python/test_qgsfieldformatters.py
+++ b/tests/src/python/test_qgsfieldformatters.py
@@ -203,6 +203,7 @@ class TestQgsRelationReferenceFieldFormatter(unittest.TestCase):
         self.assertTrue(first_layer.isValid())
         second_layer = QgsVectorLayer("none?field=pkid:integer&field=decoded:string",
                                       "second_layer", "memory")
+        second_layer.setDisplayExpression('pkid')
         self.assertTrue(second_layer.isValid())
         QgsProject.instance().addMapLayers([first_layer, second_layer])
         f = QgsFeature()
@@ -241,10 +242,10 @@ class TestQgsRelationReferenceFieldFormatter(unittest.TestCase):
         second_layer.setDisplayExpression('decoded')
         self.assertEqual(fieldFormatter.representValue(first_layer, 0, config, None, '123'), '123')
 
-        # No display expression
+        # No display expression - will default internally to the decoded string
         config = {'Relation': rel.id()}
         second_layer.setDisplayExpression(None)
-        self.assertEqual(fieldFormatter.representValue(first_layer, 0, config, None, '123'), '123')
+        self.assertEqual(fieldFormatter.representValue(first_layer, 0, config, None, '123'), 'decoded_val')
 
         # Invalid display expression
         config = {'Relation': rel.id()}
diff --git a/tests/src/python/test_qgsvectorlayer.py b/tests/src/python/test_qgsvectorlayer.py
index a51d8c2ad2e..dbcb6a09947 100644
--- a/tests/src/python/test_qgsvectorlayer.py
+++ b/tests/src/python/test_qgsvectorlayer.py
@@ -3084,9 +3084,6 @@ class TestQgsVectorLayer(unittest.TestCase, FeatureSourceTestCase):
         layer = QgsVectorLayer("Polygon?crs=epsg:2056&field=pk:int", "vl", "memory")
         self.assertEqual(layer.displayExpression(), '"pk"')
         self.assertEqual(layer.displayField(), 'pk')
-        layer = QgsVectorLayer("Polygon?crs=epsg:2056&field=pk:int&field=fid:int", "vl", "memory")
-        self.assertEqual(layer.displayExpression(), '"fid"')
-        self.assertEqual(layer.displayField(), 'fid')
         layer = QgsVectorLayer("Polygon?crs=epsg:2056&field=pk:int&field=DESCRIPTION:string&field=fid:int", "vl", "memory")
         self.assertEqual(layer.displayExpression(), '"DESCRIPTION"')
         self.assertEqual(layer.displayField(), 'DESCRIPTION')