diff --git a/src/NHibernate.Test/Async/Linq/ConstantTest.cs b/src/NHibernate.Test/Async/Linq/ConstantTest.cs
index b5f0f05037d..35e8c9bfc3d 100644
--- a/src/NHibernate.Test/Async/Linq/ConstantTest.cs
+++ b/src/NHibernate.Test/Async/Linq/ConstantTest.cs
@@ -11,6 +11,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using NHibernate.DomainModel.Northwind.Entities;
+using NHibernate.Linq.Visitors;
 using NUnit.Framework;
 using NHibernate.Linq;
 
@@ -179,7 +180,6 @@ public int GetItemValue(Product p)
 
 		// Adapted from NH-2500 first test case by Andrey Titov (file NHTest3.zip)
 		[Test]
-		[Ignore("Not fixed yet")]
 		public async Task ObjectConstantsAsync()
 		{
 			var builder = new InfoBuilder(1);
@@ -200,7 +200,6 @@ private int TestFunc(Product item, int closureValue)
 
 		// Adapted from NH-3673
 		[Test]
-		[Ignore("Not fixed yet")]
 		public async Task ConstantsInFuncCallAsync()
 		{
 			var closureVariable = 1;
diff --git a/src/NHibernate.Test/Linq/ConstantTest.cs b/src/NHibernate.Test/Linq/ConstantTest.cs
index a30118e7283..24d474951e0 100644
--- a/src/NHibernate.Test/Linq/ConstantTest.cs
+++ b/src/NHibernate.Test/Linq/ConstantTest.cs
@@ -1,6 +1,7 @@
 using System.Collections.Generic;
 using System.Linq;
 using NHibernate.DomainModel.Northwind.Entities;
+using NHibernate.Linq.Visitors;
 using NUnit.Framework;
 
 namespace NHibernate.Test.Linq
@@ -167,7 +168,6 @@ public int GetItemValue(Product p)
 
 		// Adapted from NH-2500 first test case by Andrey Titov (file NHTest3.zip)
 		[Test]
-		[Ignore("Not fixed yet")]
 		public void ObjectConstants()
 		{
 			var builder = new InfoBuilder(1);
@@ -188,7 +188,6 @@ private int TestFunc(Product item, int closureValue)
 
 		// Adapted from NH-3673
 		[Test]
-		[Ignore("Not fixed yet")]
 		public void ConstantsInFuncCall()
 		{
 			var closureVariable = 1;
@@ -201,5 +200,24 @@ public void ConstantsInFuncCall()
 			Assert.That(v1, Is.EqualTo(1), "v1");
 			Assert.That(v2, Is.EqualTo(2), "v2");
 		}
+
+		[Test]
+		public void ConstantInWhereDoesNotCauseManyKeys()
+		{
+			var q1 = (from c in db.Customers
+			          where c.CustomerId == "ALFKI"
+			          select c);
+			var q2 = (from c in db.Customers
+			          where c.CustomerId == "ANATR"
+			          select c);
+			var parameters1 = ExpressionParameterVisitor.Visit(q1.Expression, Sfi);
+			var k1 = ExpressionKeyVisitor.Visit(q1.Expression, parameters1);
+			var parameters2 = ExpressionParameterVisitor.Visit(q2.Expression, Sfi);
+			var k2 = ExpressionKeyVisitor.Visit(q2.Expression, parameters2);
+
+			Assert.That(parameters1, Has.Count.GreaterThan(0), "parameters1");
+			Assert.That(parameters2, Has.Count.GreaterThan(0), "parameters2");
+			Assert.That(k2, Is.EqualTo(k1));
+		}
 	}
 }
diff --git a/src/NHibernate/Linq/Visitors/ExpressionKeyVisitor.cs b/src/NHibernate/Linq/Visitors/ExpressionKeyVisitor.cs
index 1789357cfb0..ee053687006 100644
--- a/src/NHibernate/Linq/Visitors/ExpressionKeyVisitor.cs
+++ b/src/NHibernate/Linq/Visitors/ExpressionKeyVisitor.cs
@@ -109,19 +109,23 @@ protected override Expression VisitConstant(ConstantExpression expression)
 				{
 					_string.Append("NULL");
 				}
-				else
+				else if (expression.Value is IQueryable)
 				{
-					var value = expression.Value as IEnumerable;
-					if (value != null  && !(value is string) && !(value is IQueryable))
-					{
-						_string.Append("{");
-						_string.Append(String.Join(",", value.Cast<object>()));
-						_string.Append("}");
-					}
-					else
+					_string.Append(expression.Value);
+				}
+				else if (expression.Value is IEnumerable enumerable && !(enumerable is string))
+				{
+					_string.Append("{");
+					foreach (var value in enumerable)
 					{
-						_string.Append(expression.Value);
+						_string.Append(value).Append("#").Append(value.GetHashCode()).Append(",");
 					}
+
+					_string.Append("}");
+				}
+				else
+				{
+					_string.Append(expression.Value).Append("#").Append(expression.Value.GetHashCode());
 				}
 			}
 
@@ -163,20 +167,21 @@ protected override Expression VisitMethodCall(MethodCallExpression expression)
 		{
 			var old = insideSelectClause;
 
-			switch (expression.Method.Name)
-			{
-				case "First":
-				case "FirstOrDefault":
-				case "Single":
-				case "SingleOrDefault":
-				case "Select":
-				case "GroupBy":
-					insideSelectClause = true;
-					break;
-				default:
-					insideSelectClause = false;
-					break;
-			}
+			if (expression.Method.DeclaringType?.Namespace == "System.Linq")
+				switch (expression.Method.Name)
+				{
+					case "First":
+					case "FirstOrDefault":
+					case "Single":
+					case "SingleOrDefault":
+					case "Select":
+					case "GroupBy":
+						insideSelectClause = true;
+						break;
+					default:
+						insideSelectClause = false;
+						break;
+				}
 
 			Visit(expression.Object);
 			_string.Append('.');