Since AutoPersistenceModel can be inherited, we can't do reflection on GetType(), since it will return the derived class' type and not typeof(AutoPersistenceModel).

Author: asbjornu

src/FluentNHibernate/Automapping/AutoPersistenceModel.cs

@@ -363,10 +363,16 @@ public void Override(Type overrideType)
 					if (x.GetType().IsAssignableFrom(autoMappingType))
 					{
 						var overrideInstance = Activator.CreateInstance(overrideType);
-						GetType()
-							.GetMethod("OverrideHelper", BindingFlags.NonPublic | BindingFlags.Instance)
-							.MakeGenericMethod(entityType)
-							.Invoke(this, new [] {x, overrideInstance});
+
+						MethodInfo overrideHelperMethod = typeof(AutoPersistenceModel)
+							.GetMethod("OverrideHelper", BindingFlags.NonPublic | BindingFlags.Instance);
+						
+						if (overrideHelperMethod != null)
+						{
+							overrideHelperMethod
+								.MakeGenericMethod(entityType)
+								.Invoke(this, new[] {x, overrideInstance});
+						}
 					}
 				});
 			}