diff --git a/src/NHibernate.Test/Linq/WhereTests.cs b/src/NHibernate.Test/Linq/WhereTests.cs
index 2f8f66e5c0a..d35d9bc079e 100644
--- a/src/NHibernate.Test/Linq/WhereTests.cs
+++ b/src/NHibernate.Test/Linq/WhereTests.cs
@@ -652,6 +652,24 @@ where o is Dog
 			Assert.That(query.Count, Is.EqualTo(2));
 		}
 
+		[Test(Description = "NH-3946")]
+		public void PolymorphicSearchOnObjectTypeWithIsKeyword()
+		{
+			var query = (from o in session.Query<Animal>()
+						 where o is Mammal
+						 select o).ToList();
+
+			Assert.That(query.Count, Is.EqualTo(3));
+		}
+
+		[Test(Description = "NH-3845")]
+		public void PolymorphicSearchOnObjectTypeWithOfType()
+		{
+			var query = session.Query<Animal>().OfType<Mammal>().ToList();
+
+			Assert.That(query.Count, Is.EqualTo(3));
+		}
+
 		[Test]
 		public void BitwiseQuery()
 		{
diff --git a/src/NHibernate/Linq/Visitors/HqlGeneratorExpressionTreeVisitor.cs b/src/NHibernate/Linq/Visitors/HqlGeneratorExpressionTreeVisitor.cs
index 9b80a446a4b..4bdbffa05a7 100644
--- a/src/NHibernate/Linq/Visitors/HqlGeneratorExpressionTreeVisitor.cs
+++ b/src/NHibernate/Linq/Visitors/HqlGeneratorExpressionTreeVisitor.cs
@@ -147,9 +147,54 @@ protected HqlTreeNode VisitExpression(Expression expression)
 
 		private HqlTreeNode VisitTypeBinaryExpression(TypeBinaryExpression expression)
 		{
+			return BuildOfType(expression.Expression, expression.TypeOperand);
+		}
+
+		internal HqlBooleanExpression BuildOfType(Expression expression, System.Type type)
+		{
+			var sessionFactory = _parameters.SessionFactory;
+			var meta = sessionFactory.GetClassMetadata(type) as Persister.Entity.AbstractEntityPersister;
+			if (meta != null && !meta.IsExplicitPolymorphism)
+			{
+				//Adapted the logic found in SingleTableEntityPersister.DiscriminatorFilterFragment
+				var nodes = meta
+					.SubclassClosure
+					.Select(typeName => (NHibernate.Persister.Entity.IQueryable) sessionFactory.GetEntityPersister(typeName))
+					.Where(persister => !persister.IsAbstract)
+					.Select(persister => _hqlTreeBuilder.Ident(persister.EntityName))
+					.ToList();
+
+				if (nodes.Count == 1)
+				{
+					return _hqlTreeBuilder.Equality(
+						_hqlTreeBuilder.Dot(Visit(expression).AsExpression(), _hqlTreeBuilder.Class()),
+						nodes[0]);
+				}
+
+				if (nodes.Count > 1)
+				{
+					return _hqlTreeBuilder.In(
+						_hqlTreeBuilder.Dot(
+							Visit(expression).AsExpression(),
+							_hqlTreeBuilder.Class()),
+						_hqlTreeBuilder.ExpressionSubTreeHolder(nodes));
+				}
+
+				if (nodes.Count == 0)
+				{
+					const string abstractClassWithNoSubclassExceptionMessageTemplate =
+@"The class {0} can't be instatiated and does not have mapped subclasses; 
+possible solutions:
+- don't map the abstract class
+- map its subclasses.";
+
+					throw new NotSupportedException(string.Format(abstractClassWithNoSubclassExceptionMessageTemplate, meta.EntityName));
+				}
+			}
+
 			return _hqlTreeBuilder.Equality(
-				_hqlTreeBuilder.Dot(Visit(expression.Expression).AsExpression(), _hqlTreeBuilder.Class()),
-				_hqlTreeBuilder.Ident(expression.TypeOperand.FullName));
+				_hqlTreeBuilder.Dot(Visit(expression).AsExpression(), _hqlTreeBuilder.Class()),
+				_hqlTreeBuilder.Ident(type.FullName));
 		}
 
 		protected HqlTreeNode VisitNhStar(NhStarExpression expression)
diff --git a/src/NHibernate/Linq/Visitors/ResultOperatorProcessors/ProcessOfType.cs b/src/NHibernate/Linq/Visitors/ResultOperatorProcessors/ProcessOfType.cs
index 7c9613df7a8..4da0271fee2 100644
--- a/src/NHibernate/Linq/Visitors/ResultOperatorProcessors/ProcessOfType.cs
+++ b/src/NHibernate/Linq/Visitors/ResultOperatorProcessors/ProcessOfType.cs
@@ -1,24 +1,17 @@
-using System.Linq.Expressions;
-using NHibernate.Hql.Ast;
-using Remotion.Linq.Clauses.ResultOperators;
+using Remotion.Linq.Clauses.ResultOperators;
 
 namespace NHibernate.Linq.Visitors.ResultOperatorProcessors
 {
 	public class ProcessOfType : IResultOperatorProcessor<OfTypeResultOperator>
 	{
-		#region IResultOperatorProcessor<OfTypeResultOperator> Members
-
 		public void Process(OfTypeResultOperator resultOperator, QueryModelVisitor queryModelVisitor, IntermediateHqlTree tree)
 		{
-		    Expression source = queryModelVisitor.Model.SelectClause.GetOutputDataInfo().ItemExpression;
+			var source = queryModelVisitor.Model.SelectClause.GetOutputDataInfo().ItemExpression;
 
-			tree.AddWhereClause(tree.TreeBuilder.Equality(
-				tree.TreeBuilder.Dot(
-					HqlGeneratorExpressionTreeVisitor.Visit(source, queryModelVisitor.VisitorParameters).AsExpression(),
-					tree.TreeBuilder.Class()),
-				tree.TreeBuilder.Ident(resultOperator.SearchedItemType.FullName)));
-		}
+			var expression = new HqlGeneratorExpressionTreeVisitor(queryModelVisitor.VisitorParameters)
+				.BuildOfType(source, resultOperator.SearchedItemType);
 
-		#endregion
+			tree.AddWhereClause(expression);
+		}
 	}
 }
\ No newline at end of file
diff --git a/src/NHibernate/Persister/Entity/AbstractEntityPersister.cs b/src/NHibernate/Persister/Entity/AbstractEntityPersister.cs
index f817f5647d9..c0b98bb0eeb 100644
--- a/src/NHibernate/Persister/Entity/AbstractEntityPersister.cs
+++ b/src/NHibernate/Persister/Entity/AbstractEntityPersister.cs
@@ -1013,6 +1013,7 @@ public int[] NaturalIdentifierProperties
 		public abstract string[] ConstraintOrderedTableNameClosure { get;}
 		public abstract string DiscriminatorSQLValue { get;}
 		public abstract object DiscriminatorValue { get;}
+		public abstract string[] SubclassClosure { get; }
 		public abstract string[] PropertySpaces { get;}
 
 		protected virtual void AddDiscriminatorToInsert(SqlInsertBuilder insert) { }
diff --git a/src/NHibernate/Persister/Entity/JoinedSubclassEntityPersister.cs b/src/NHibernate/Persister/Entity/JoinedSubclassEntityPersister.cs
index 63aae0dcc44..c1510d9409c 100644
--- a/src/NHibernate/Persister/Entity/JoinedSubclassEntityPersister.cs
+++ b/src/NHibernate/Persister/Entity/JoinedSubclassEntityPersister.cs
@@ -345,6 +345,11 @@ public override object DiscriminatorValue
 			get { return discriminatorValue; }
 		}
 
+		public override string[] SubclassClosure
+		{
+			get { return subclassClosure; }
+		}
+
 		public override string[] PropertySpaces
 		{
 			get
diff --git a/src/NHibernate/Persister/Entity/SingleTableEntityPersister.cs b/src/NHibernate/Persister/Entity/SingleTableEntityPersister.cs
index 0131d84aafa..7ce36aaa03f 100644
--- a/src/NHibernate/Persister/Entity/SingleTableEntityPersister.cs
+++ b/src/NHibernate/Persister/Entity/SingleTableEntityPersister.cs
@@ -424,7 +424,7 @@ public override object DiscriminatorValue
 			get { return discriminatorValue; }
 		}
 
-		public virtual string[] SubclassClosure
+		public override string[] SubclassClosure
 		{
 			get { return subclassClosure; }
 		}
@@ -618,7 +618,7 @@ private string DiscriminatorFilterFragment(string alias)
 @"The class {0} can't be instatiated and does not have mapped subclasses; 
 possible solutions:
 - don't map the abstract class
-- map the its subclasses.";
+- map its subclasses.";
 
 			if (NeedsDiscriminator)
 			{
diff --git a/src/NHibernate/Persister/Entity/UnionSubclassEntityPersister.cs b/src/NHibernate/Persister/Entity/UnionSubclassEntityPersister.cs
index a1a3d5267bf..ad93180dc2d 100644
--- a/src/NHibernate/Persister/Entity/UnionSubclassEntityPersister.cs
+++ b/src/NHibernate/Persister/Entity/UnionSubclassEntityPersister.cs
@@ -186,7 +186,7 @@ public override object DiscriminatorValue
 			get { return discriminatorValue; }
 		}
 
-		public string[] SubclassClosure
+		public override string[] SubclassClosure
 		{
 			get { return subclassClosure; }
 		}