Index: Ringtoets/ClosingStructures/src/Ringtoets.ClosingStructures.Data/ClosingStructuresInput.cs
===================================================================
diff -u -r38cd5da3aefa4ea677c2b1796cbb9b831d41fe3c -r25b4c6adf1c80f2e25a69a172679362938dc8efb
--- Ringtoets/ClosingStructures/src/Ringtoets.ClosingStructures.Data/ClosingStructuresInput.cs	(.../ClosingStructuresInput.cs)	(revision 38cd5da3aefa4ea677c2b1796cbb9b831d41fe3c)
+++ Ringtoets/ClosingStructures/src/Ringtoets.ClosingStructures.Data/ClosingStructuresInput.cs	(.../ClosingStructuresInput.cs)	(revision 25b4c6adf1c80f2e25a69a172679362938dc8efb)
@@ -21,6 +21,7 @@
 
 using System;
 using Core.Common.Base.Data;
+using Ringtoets.ClosingStructures.Data.Properties;
 using Ringtoets.Common.Data.Probabilistics;
 using Ringtoets.Common.Data.Probability;
 using Ringtoets.Common.Data.Structures;
@@ -35,9 +36,11 @@
     {
         private const int deviationWaveDirectionNumberOfDecimals = 2;
 
-        private static readonly Range<RoundedDouble> deviationWaveDirectionValidityRange = new Range<RoundedDouble>(new RoundedDouble(deviationWaveDirectionNumberOfDecimals, -360),
-                                                                                                                    new RoundedDouble(deviationWaveDirectionNumberOfDecimals, 360));
+        private static readonly Range<RoundedDouble> deviationWaveDirectionValidityRange = new Range<RoundedDouble>(
+            new RoundedDouble(deviationWaveDirectionNumberOfDecimals, -360), new RoundedDouble(deviationWaveDirectionNumberOfDecimals, 360));
 
+        private static readonly Range<int> identicalAperturesValidityRange = new Range<int>(1, int.MaxValue);
+
         private NormalDistribution thresholdHeightOpenWeir;
         private NormalDistribution drainCoefficient;
         private LogNormalDistribution areaFlowApertures;
@@ -48,6 +51,7 @@
         private double failureProbabilityReparation;
         private double probabilityOpenStructureBeforeFlooding;
         private RoundedDouble deviationWaveDirection;
+        private int identicalApertures;
 
         /// <summary>
         /// Creates a new instance of the <see cref="ClosingStructuresInput"/> class.
@@ -172,7 +176,7 @@
                 StandardDeviation = RoundedDouble.NaN
             };
 
-            IdenticalApertures = 0;
+            IdenticalApertures = 1;
             InflowModelType = 0;
         }
 
@@ -331,7 +335,23 @@
         /// <summary>
         /// Gets or sets the amount of identical apertures.
         /// </summary>
-        public int IdenticalApertures { get; set; }
+        /// <exception cref="ArgumentOutOfRangeException">Thrown when the value is smaller than 1.</exception>
+        public int IdenticalApertures
+        {
+            get
+            {
+                return identicalApertures;
+            }
+            set
+            {
+                if (!identicalAperturesValidityRange.InRange(value))
+                {
+                    throw new ArgumentOutOfRangeException(nameof(IdenticalApertures),
+                                                          Resources.ClosingStructuresInput_IdenticalApertures_must_be_equal_or_greater_to_one);
+                }
+                identicalApertures = value;
+            }
+        }
 
         /// <summary>
         /// Gets or sets the level of crest of the structures that are not closed.