diff --git a/src/NHibernate.Test/Async/NHSpecificTest/GH1760/MapKeyFormulaFixture.cs b/src/NHibernate.Test/Async/NHSpecificTest/GH1760/MapKeyFormulaFixture.cs
new file mode 100644
index 00000000000..2234a7a5b89
--- /dev/null
+++ b/src/NHibernate.Test/Async/NHSpecificTest/GH1760/MapKeyFormulaFixture.cs
@@ -0,0 +1,83 @@
+//------------------------------------------------------------------------------
+// <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 NUnit.Framework;
+using NHibernate.Linq;
+
+namespace NHibernate.Test.NHSpecificTest.GH1760
+{
+	using System.Threading.Tasks;
+	[TestFixture]
+	public class MapKeyFormulasFixtureAsync : BugTestCase
+	{
+		protected override void OnSetUp()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				var bob =
+					new User
+					{
+						Name = "Bob"
+					};
+				session.Save(bob);
+
+				var sally =
+					new User
+					{
+						Name = "Sally"
+					};
+				session.Save(sally);
+
+				var g1 = new Group { Name = "Salesperson" };
+				session.Save(g1);
+
+				g1.UsersByName.Add(bob.Name, bob);
+				g1.UsersByName.Add(sally.Name, sally);
+
+				transaction.Commit();
+			}
+		}
+
+		protected override void OnTearDown()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				// HQL "delete from" does not handle foreign key order.
+				session.Delete("from User");
+				session.CreateQuery("delete System.Object").ExecuteUpdate();
+
+				transaction.Commit();
+			}
+		}
+
+		[Test]
+		public async Task CheckMapKeyAsync()
+		{
+			using (var session = OpenSession())
+			using (var tx = session.BeginTransaction())
+			{
+				var salesperson = await (session.Query<Group>().SingleAsync(u => u.Name == "Salesperson"));
+
+				Assert.That(
+					salesperson.UsersByName,
+					Has.Count.EqualTo(2)
+					   .And.One.Property(nameof(KeyValuePair<string, User>.Key)).EqualTo("Bob")
+					   .And.One.Property(nameof(KeyValuePair<string, User>.Key)).EqualTo("Sally"));
+
+				await (tx.CommitAsync());
+			}
+		}
+	}
+}
diff --git a/src/NHibernate.Test/NHSpecificTest/GH1760/Domain.cs b/src/NHibernate.Test/NHSpecificTest/GH1760/Domain.cs
new file mode 100644
index 00000000000..576bc9c7900
--- /dev/null
+++ b/src/NHibernate.Test/NHSpecificTest/GH1760/Domain.cs
@@ -0,0 +1,19 @@
+using System;
+using System.Collections.Generic;
+
+namespace NHibernate.Test.NHSpecificTest.GH1760
+{
+	class User
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
+	}
+
+	class Group
+	{
+		public virtual Guid Id { get; set; }
+		public virtual string Name { get; set; }
+
+		public virtual IDictionary<string, User> UsersByName { get; set; } = new Dictionary<string, User>();
+	}
+}
diff --git a/src/NHibernate.Test/NHSpecificTest/GH1760/MapKeyFormulaFixture.cs b/src/NHibernate.Test/NHSpecificTest/GH1760/MapKeyFormulaFixture.cs
new file mode 100644
index 00000000000..9b7d9a828a3
--- /dev/null
+++ b/src/NHibernate.Test/NHSpecificTest/GH1760/MapKeyFormulaFixture.cs
@@ -0,0 +1,71 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using NUnit.Framework;
+
+namespace NHibernate.Test.NHSpecificTest.GH1760
+{
+	[TestFixture]
+	public class MapKeyFormulasFixture : BugTestCase
+	{
+		protected override void OnSetUp()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				var bob =
+					new User
+					{
+						Name = "Bob"
+					};
+				session.Save(bob);
+
+				var sally =
+					new User
+					{
+						Name = "Sally"
+					};
+				session.Save(sally);
+
+				var g1 = new Group { Name = "Salesperson" };
+				session.Save(g1);
+
+				g1.UsersByName.Add(bob.Name, bob);
+				g1.UsersByName.Add(sally.Name, sally);
+
+				transaction.Commit();
+			}
+		}
+
+		protected override void OnTearDown()
+		{
+			using (var session = OpenSession())
+			using (var transaction = session.BeginTransaction())
+			{
+				// HQL "delete from" does not handle foreign key order.
+				session.Delete("from User");
+				session.CreateQuery("delete System.Object").ExecuteUpdate();
+
+				transaction.Commit();
+			}
+		}
+
+		[Test]
+		public void CheckMapKey()
+		{
+			using (var session = OpenSession())
+			using (var tx = session.BeginTransaction())
+			{
+				var salesperson = session.Query<Group>().Single(u => u.Name == "Salesperson");
+
+				Assert.That(
+					salesperson.UsersByName,
+					Has.Count.EqualTo(2)
+					   .And.One.Property(nameof(KeyValuePair<string, User>.Key)).EqualTo("Bob")
+					   .And.One.Property(nameof(KeyValuePair<string, User>.Key)).EqualTo("Sally"));
+
+				tx.Commit();
+			}
+		}
+	}
+}
diff --git a/src/NHibernate.Test/NHSpecificTest/GH1760/Mappings.hbm.xml b/src/NHibernate.Test/NHSpecificTest/GH1760/Mappings.hbm.xml
new file mode 100644
index 00000000000..309cf3b759c
--- /dev/null
+++ b/src/NHibernate.Test/NHSpecificTest/GH1760/Mappings.hbm.xml
@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" assembly="NHibernate.Test"
+                   namespace="NHibernate.Test.NHSpecificTest.GH1760">
+
+  <class name="User" table="`User`">
+    <id name="Id" generator="guid.comb"/>
+    <property name="Name"/>
+  </class>
+
+  <class name="Group" table="`Group`">
+    <id name="Id" generator="guid.comb"/>
+
+    <property name="Name"/>
+
+    <map name="UsersByName">
+      <key column="GroupId"/>
+      <map-key type="string">
+        <formula>Name</formula>
+      </map-key>
+      <one-to-many class="User"/>
+    </map>
+  </class>
+
+</hibernate-mapping>
diff --git a/src/NHibernate/Persister/Collection/OneToManyPersister.cs b/src/NHibernate/Persister/Collection/OneToManyPersister.cs
index f87b2e60199..f88e1bd2bdf 100644
--- a/src/NHibernate/Persister/Collection/OneToManyPersister.cs
+++ b/src/NHibernate/Persister/Collection/OneToManyPersister.cs
@@ -76,7 +76,7 @@ protected override SqlCommandInfo GenerateDeleteString()
 				update.SetJoin(ownerPersister.TableName, KeyColumnNames, KeyType, JoinColumnNames, ownerPersister.GetPropertyColumnNames(CollectionType.LHSPropertyName));
 			}
 
-			if (HasIndex)
+			if (HasIndex && !indexContainsFormula)
 				update.AddColumns(IndexColumnNames, "null");
 
 			if (HasWhere)