Index: Ringtoets/Common/src/Ringtoets.Common.IO/Readers/CalculationConfigurationReader.cs
===================================================================
diff -u -r19c6f79760da1309b849ed421990431f969c4f78 -r9c07cee1e1a14a24e2d949bf9b32f294f0988f1f
--- Ringtoets/Common/src/Ringtoets.Common.IO/Readers/CalculationConfigurationReader.cs	(.../CalculationConfigurationReader.cs)	(revision 19c6f79760da1309b849ed421990431f969c4f78)
+++ Ringtoets/Common/src/Ringtoets.Common.IO/Readers/CalculationConfigurationReader.cs	(.../CalculationConfigurationReader.cs)	(revision 9c07cee1e1a14a24e2d949bf9b32f294f0988f1f)
@@ -167,14 +167,11 @@
                 throw new ArgumentException($"'{nameof(mainSchemaDefinition)}' does not reference the default schema '{defaultSchemaName}'.");
             }
 
-            var combinedXmlSchemaDefinition = new CombinedXmlSchemaDefinition(
-                mainSchemaDefinition,
-                nestedSchemaDefinitions.Concat(new[]
-                                       {
-                                           new KeyValuePair<string, string>(defaultSchemaName, Resources.ConfiguratieSchema)
-                                       })
-                                       .ToDictionary(kv => kv.Key, kv => kv.Value));
+            IDictionary<string, string> extendedNestedSchemaDefinitions = new Dictionary<string, string>(nestedSchemaDefinitions);
+            extendedNestedSchemaDefinitions.Add(defaultSchemaName, Resources.ConfiguratieSchema);
 
+            var combinedXmlSchemaDefinition = new CombinedXmlSchemaDefinition(mainSchemaDefinition, extendedNestedSchemaDefinitions);
+
             try
             {
                 combinedXmlSchemaDefinition.Validate(document);