Index: Ringtoets/Common/src/Ringtoets.Common.IO/ReferenceLines/ReferenceLineImporter.cs
===================================================================
diff -u -r75b7759b5721edc5e0458229e2d727723f527391 -r9fe132b5fdbde6b462235fccc39bccd521093a4f
--- Ringtoets/Common/src/Ringtoets.Common.IO/ReferenceLines/ReferenceLineImporter.cs	(.../ReferenceLineImporter.cs)	(revision 75b7759b5721edc5e0458229e2d727723f527391)
+++ Ringtoets/Common/src/Ringtoets.Common.IO/ReferenceLines/ReferenceLineImporter.cs	(.../ReferenceLineImporter.cs)	(revision 9fe132b5fdbde6b462235fccc39bccd521093a4f)
@@ -45,13 +45,17 @@
         /// <param name="updateHandler">The object responsible for updating the
         /// <see cref="ReferenceLine"/>.</param>
         /// <param name="filePath">The path to the file to import from.</param>
-        /// <exception cref="ArgumentNullException">Thrown when <paramref name="importTarget"/>
-        /// or <paramref name="filePath"/> is <c>null</c>.</exception>
+        /// <exception cref="ArgumentNullException">Thrown when any parameter is <c>null</c>.</exception>
         public ReferenceLineImporter(ReferenceLine importTarget,
                                      IReferenceLineUpdateHandler updateHandler,
                                      string filePath)
             : base(filePath, importTarget)
         {
+            if (updateHandler == null)
+            {
+                throw new ArgumentNullException(nameof(updateHandler));
+            }
+
             this.updateHandler = updateHandler;
         }
 
Index: Ringtoets/Common/test/Ringtoets.Common.IO.Test/ReferenceLines/ReferenceLineImporterTest.cs
===================================================================
diff -u -rd7ec57b0867842019cbd48a736db96e2509ed0f4 -r9fe132b5fdbde6b462235fccc39bccd521093a4f
--- Ringtoets/Common/test/Ringtoets.Common.IO.Test/ReferenceLines/ReferenceLineImporterTest.cs	(.../ReferenceLineImporterTest.cs)	(revision d7ec57b0867842019cbd48a736db96e2509ed0f4)
+++ Ringtoets/Common/test/Ringtoets.Common.IO.Test/ReferenceLines/ReferenceLineImporterTest.cs	(.../ReferenceLineImporterTest.cs)	(revision 9fe132b5fdbde6b462235fccc39bccd521093a4f)
@@ -54,6 +54,17 @@
         }
 
         [Test]
+        public void Constructor_UpdateHandlerNull_ThrowsArgumentNullException()
+        {
+            // Call
+            TestDelegate call = () => new ReferenceLineImporter(new ReferenceLine(), null, "");
+
+            // Assert
+            var exception = Assert.Throws<ArgumentNullException>(call);
+            Assert.AreEqual("updateHandler", exception.ParamName);
+        }
+
+        [Test]
         public void Constructor_ExpectedValues()
         {
             // Setup
@@ -438,7 +449,6 @@
             observable2.Expect(o => o.NotifyObservers());
 
             var handler = mocks.Stub<IReferenceLineUpdateHandler>();
-            handler.Stub(h => h.ConfirmUpdate()).Return(true);
             handler.Expect(h => h.Update(Arg<ReferenceLine>.Is.Same(referenceLine),
                                          Arg<ReferenceLine>.Is.NotNull))
                    .Return(new[]