Index: Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Utils/AssignUnassignCalculations.cs
===================================================================
diff -u -r9268ebeb017950ef5cc9d9d99f27bbbb91d5b19f -r3d83d16a31cb6fdc70e7b0e2c0e7f7915bcaa465
--- Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Utils/AssignUnassignCalculations.cs	(.../AssignUnassignCalculations.cs)	(revision 9268ebeb017950ef5cc9d9d99f27bbbb91d5b19f)
+++ Ringtoets/GrassCoverErosionInwards/src/Ringtoets.GrassCoverErosionInwards.Utils/AssignUnassignCalculations.cs	(.../AssignUnassignCalculations.cs)	(revision 3d83d16a31cb6fdc70e7b0e2c0e7f7915bcaa465)
@@ -65,7 +65,7 @@
         /// </summary>
         /// <param name="sectionResults">The <see cref="GrassCoverErosionInwardsFailureMechanismSectionResult"/> objects.</param>
         /// <param name="calculation">The <see cref="GrassCoverErosionInwardsCalculation"/>.</param>
-        /// <param name="calculations"></param>
+        /// <param name="calculations">All the remaining calculations after deletion of the <paramref name="calculation"/>.</param>
         /// <exception cref="ArgumentNullException">When any input parameter is <c>null</c>.</exception>
         public static void Delete(
             IEnumerable<GrassCoverErosionInwardsFailureMechanismSectionResult> sectionResults, 
@@ -80,6 +80,10 @@
             {
                 throw new ArgumentNullException("calculation");
             }
+            if (calculations == null)
+            {
+                throw new ArgumentNullException("calculations");
+            }
 
             var sectionResultsArray = sectionResults.ToArray();
 
@@ -94,16 +98,14 @@
             GrassCoverErosionInwardsCalculation calculation, Dictionary<string, 
             IList<GrassCoverErosionInwardsCalculation>> calculationsPerSegmentName)
         {
-            var sectionsContainingOneCalculation = calculationsPerSegmentName.Where(kvp => kvp.Value.Count == 1).ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
-
             IEnumerable<GrassCoverErosionInwardsFailureMechanismSectionResult> sectionResultsUsingCalculation =
                 sectionResults.Where(sr => sr.Calculation != null && sr.Calculation.Equals(calculation));
             foreach (var sectionResult in sectionResultsUsingCalculation)
             {
                 string sectionName = sectionResult.Section.Name;
-                if (sectionsContainingOneCalculation.ContainsKey(sectionName))
+                if (calculationsPerSegmentName.ContainsKey(sectionName) && calculationsPerSegmentName[sectionName].Count == 1)
                 {
-                    sectionResult.Calculation = sectionsContainingOneCalculation[sectionName][0];
+                    sectionResult.Calculation = calculationsPerSegmentName[sectionName].Single();
                     continue;
                 }
                 sectionResult.Calculation = null;
Index: Ringtoets/GrassCoverErosionInwards/test/Ringtoets.GrassCoverErosionInwards.Utils.Test/AssignUnassignCalculationsTest.cs
===================================================================
diff -u -r9268ebeb017950ef5cc9d9d99f27bbbb91d5b19f -r3d83d16a31cb6fdc70e7b0e2c0e7f7915bcaa465
--- Ringtoets/GrassCoverErosionInwards/test/Ringtoets.GrassCoverErosionInwards.Utils.Test/AssignUnassignCalculationsTest.cs	(.../AssignUnassignCalculationsTest.cs)	(revision 9268ebeb017950ef5cc9d9d99f27bbbb91d5b19f)
+++ Ringtoets/GrassCoverErosionInwards/test/Ringtoets.GrassCoverErosionInwards.Utils.Test/AssignUnassignCalculationsTest.cs	(.../AssignUnassignCalculationsTest.cs)	(revision 3d83d16a31cb6fdc70e7b0e2c0e7f7915bcaa465)
@@ -170,7 +170,7 @@
             var calculation = new GrassCoverErosionInwardsCalculation();
 
             // Call
-            TestDelegate call = () => AssignUnassignCalculations.Update(null, calculation);
+            TestDelegate call = () => AssignUnassignCalculations.Delete(null, calculation, Enumerable.Empty<GrassCoverErosionInwardsCalculation>());
 
             // Assert
             string paramName = Assert.Throws<ArgumentNullException>(call).ParamName;
@@ -184,14 +184,28 @@
             var failureMechanism = new GrassCoverErosionInwardsFailureMechanism();
 
             // Call
-            TestDelegate call = () => AssignUnassignCalculations.Update(failureMechanism.SectionResults, null);
+            TestDelegate call = () => AssignUnassignCalculations.Delete(failureMechanism.SectionResults, null, Enumerable.Empty<GrassCoverErosionInwardsCalculation>());
 
             // Assert
             string paramName = Assert.Throws<ArgumentNullException>(call).ParamName;
             Assert.AreEqual("calculation", paramName);
         }
 
         [Test]
+        public void Delete_NullCalculations_ThrowsArgumentNullException()
+        {
+            // Setup
+            var failureMechanism = new GrassCoverErosionInwardsFailureMechanism();
+
+            // Call
+            TestDelegate call = () => AssignUnassignCalculations.Delete(failureMechanism.SectionResults, new GrassCoverErosionInwardsCalculation(),  null);
+
+            // Assert
+            string paramName = Assert.Throws<ArgumentNullException>(call).ParamName;
+            Assert.AreEqual("calculations", paramName);
+        }
+
+        [Test]
         public void Delete_RemoveCalculationAssignedToSectionResult_SectionResultCalculationNull()
         {
             // Setup
@@ -221,7 +235,7 @@
             AssignUnassignCalculations.Delete(failureMechanism.SectionResults, calculation, Enumerable.Empty<GrassCoverErosionInwardsCalculation>());
 
             // Assert
-            Assert.IsNull(failureMechanism.SectionResults.First().Calculation);
+            Assert.IsNull(failureMechanism.SectionResults.Single().Calculation);
         }
 
         [Test]
@@ -258,7 +272,7 @@
                 new Point2D(0.0, 0.0), new Point2D(1.1, 1.1)
             }));
 
-            failureMechanism.SectionResults.First().Calculation = calculation1;
+            failureMechanism.SectionResults.Single().Calculation = calculation1;
 
             var remainingCalculations = new[]
             {
@@ -269,7 +283,7 @@
             AssignUnassignCalculations.Delete(failureMechanism.SectionResults, calculation1, remainingCalculations);
 
             // Assert
-            Assert.AreSame(calculation2, failureMechanism.SectionResults.First().Calculation);
+            Assert.AreSame(calculation2, failureMechanism.SectionResults.Single().Calculation);
         }
     }
 }
\ No newline at end of file