diff --git a/src/NHibernate.Test/Async/Linq/ByMethod/MappedAsTests.cs b/src/NHibernate.Test/Async/Linq/ByMethod/MappedAsTests.cs
new file mode 100644
index 00000000000..af79cd5f272
--- /dev/null
+++ b/src/NHibernate.Test/Async/Linq/ByMethod/MappedAsTests.cs
@@ -0,0 +1,52 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by AsyncGenerator.
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using NHibernate.Linq;
+using NUnit.Framework;
+
+namespace NHibernate.Test.Linq.ByMethod
+{
+	using System.Threading.Tasks;
+	[TestFixture]
+	public class MappedAsTestsAsync : LinqTestCase
+	{
+		[Test]
+		public async Task WithUnaryExpressionAsync()
+		{
+			var num = 1;
+			await (db.Orders.Where(o => o.Freight == (-num).MappedAs(NHibernateUtil.Decimal)).ToListAsync());
+			await (db.Orders.Where(o => o.Freight == ((decimal) num).MappedAs(NHibernateUtil.Decimal)).ToListAsync());
+			await (db.Orders.Where(o => o.Freight == ((decimal?) (decimal) num).MappedAs(NHibernateUtil.Decimal)).ToListAsync());
+		}
+
+		[Test]
+		public async Task WithNewExpressionAsync()
+		{
+			var num = 1;
+			await (db.Orders.Where(o => o.Freight == new decimal(num).MappedAs(NHibernateUtil.Decimal)).ToListAsync());
+		}
+
+		[Test]
+		public async Task WithMethodCallExpressionAsync()
+		{
+			var num = 1;
+			await (db.Orders.Where(o => o.Freight == GetDecimal(num).MappedAs(NHibernateUtil.Decimal)).ToListAsync());
+		}
+
+		private decimal GetDecimal(int number)
+		{
+			return number;
+		}
+	}
+}
diff --git a/src/NHibernate.Test/Linq/ByMethod/MappedAsTests.cs b/src/NHibernate.Test/Linq/ByMethod/MappedAsTests.cs
new file mode 100644
index 00000000000..f87cff8af92
--- /dev/null
+++ b/src/NHibernate.Test/Linq/ByMethod/MappedAsTests.cs
@@ -0,0 +1,41 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using NHibernate.Linq;
+using NUnit.Framework;
+
+namespace NHibernate.Test.Linq.ByMethod
+{
+	[TestFixture]
+	public class MappedAsTests : LinqTestCase
+	{
+		[Test]
+		public void WithUnaryExpression()
+		{
+			var num = 1;
+			db.Orders.Where(o => o.Freight == (-num).MappedAs(NHibernateUtil.Decimal)).ToList();
+			db.Orders.Where(o => o.Freight == ((decimal) num).MappedAs(NHibernateUtil.Decimal)).ToList();
+			db.Orders.Where(o => o.Freight == ((decimal?) (decimal) num).MappedAs(NHibernateUtil.Decimal)).ToList();
+		}
+
+		[Test]
+		public void WithNewExpression()
+		{
+			var num = 1;
+			db.Orders.Where(o => o.Freight == new decimal(num).MappedAs(NHibernateUtil.Decimal)).ToList();
+		}
+
+		[Test]
+		public void WithMethodCallExpression()
+		{
+			var num = 1;
+			db.Orders.Where(o => o.Freight == GetDecimal(num).MappedAs(NHibernateUtil.Decimal)).ToList();
+		}
+
+		private decimal GetDecimal(int number)
+		{
+			return number;
+		}
+	}
+}
diff --git a/src/NHibernate/Linq/Visitors/ExpressionParameterVisitor.cs b/src/NHibernate/Linq/Visitors/ExpressionParameterVisitor.cs
index c0658d4312c..76467a8035e 100644
--- a/src/NHibernate/Linq/Visitors/ExpressionParameterVisitor.cs
+++ b/src/NHibernate/Linq/Visitors/ExpressionParameterVisitor.cs
@@ -70,7 +70,7 @@ protected override Expression VisitMethodCall(MethodCallExpression expression)
 			{
 				var rawParameter = Visit(expression.Arguments[0]);
 				// TODO 6.0: Remove below code and return expression as this logic is now inside ConstantTypeLocator
-				var parameter = rawParameter as ConstantExpression;
+				var parameter = ParameterTypeLocator.UnwrapUnary(rawParameter) as ConstantExpression;
 				var type = expression.Arguments[1] as ConstantExpression;
 				if (parameter == null)
 					throw new HibernateException(
@@ -83,7 +83,7 @@ protected override Expression VisitMethodCall(MethodCallExpression expression)
 
 				_parameters[parameter].Type = (IType)type.Value;
 
-				return parameter;
+				return rawParameter;
 			}
 
 			var method = expression.Method.IsGenericMethod
diff --git a/src/NHibernate/Linq/Visitors/ParameterTypeLocator.cs b/src/NHibernate/Linq/Visitors/ParameterTypeLocator.cs
index 7e7d5834272..ccc950d57f2 100644
--- a/src/NHibernate/Linq/Visitors/ParameterTypeLocator.cs
+++ b/src/NHibernate/Linq/Visitors/ParameterTypeLocator.cs
@@ -228,7 +228,7 @@ protected override Expression VisitMethodCall(MethodCallExpression node)
 				if (VisitorUtil.IsMappedAs(node.Method))
 				{
 					var rawParameter = Visit(node.Arguments[0]);
-					var parameter = rawParameter as ConstantExpression;
+					var parameter = UnwrapUnary(rawParameter) as ConstantExpression;
 					var type = node.Arguments[1] as ConstantExpression;
 					if (parameter == null)
 						throw new HibernateException(
@@ -405,7 +405,7 @@ private bool IsDynamicMember(Expression expression)
 		/// </summary>
 		/// <param name="expression">The expression to unwrap.</param>
 		/// <returns>The unwrapped expression.</returns>
-		private static Expression UnwrapUnary(Expression expression)
+		internal static Expression UnwrapUnary(Expression expression)
 		{
 			while (expression is UnaryExpression unaryExpression)
 			{