diff --git a/MoreLinq.Test/ZipLongestTest.cs b/MoreLinq.Test/ZipLongestTest.cs
index f61470013..5b4e74789 100644
--- a/MoreLinq.Test/ZipLongestTest.cs
+++ b/MoreLinq.Test/ZipLongestTest.cs
@@ -15,6 +15,8 @@
 // limitations under the License.
 #endregion
 
+#nullable enable
+
 namespace MoreLinq.Test
 {
     using System;
@@ -31,13 +33,13 @@ public class ZipLongestTest
         public static readonly IEnumerable<ITestCaseData> TestData =
             from e in new[]
             {
-                new { A = Seq<int>(  ), B = Seq("foo", "bar", "baz"), Result = Seq((0, "foo"), (0, "bar"), (0, "baz")) },
-                new { A = Seq(1      ), B = Seq("foo", "bar", "baz"), Result = Seq((1, "foo"), (0, "bar"), (0, "baz")) },
-                new { A = Seq(1, 2   ), B = Seq("foo", "bar", "baz"), Result = Seq((1, "foo"), (2, "bar"), (0, "baz")) },
-                new { A = Seq(1, 2, 3), B = Seq<string>(           ), Result = Seq((1, null ), (2, null ), (3, (string) null)) },
-                new { A = Seq(1, 2, 3), B = Seq("foo"              ), Result = Seq((1, "foo"), (2, null ), (3, null )) },
-                new { A = Seq(1, 2, 3), B = Seq("foo", "bar"       ), Result = Seq((1, "foo"), (2, "bar"), (3, null )) },
-                new { A = Seq(1, 2, 3), B = Seq("foo", "bar", "baz"), Result = Seq((1, "foo"), (2, "bar"), (3, "baz")) },
+                new { A = Seq<int>(  ), B = Seq("foo", "bar", "baz"), Result = Seq<(int, string?)>((0, "foo"), (0, "bar"), (0, "baz")) },
+                new { A = Seq(1      ), B = Seq("foo", "bar", "baz"), Result = Seq<(int, string?)>((1, "foo"), (0, "bar"), (0, "baz")) },
+                new { A = Seq(1, 2   ), B = Seq("foo", "bar", "baz"), Result = Seq<(int, string?)>((1, "foo"), (2, "bar"), (0, "baz")) },
+                new { A = Seq(1, 2, 3), B = Seq<string>(           ), Result = Seq<(int, string?)>((1, null ), (2, null ), (3, null )) },
+                new { A = Seq(1, 2, 3), B = Seq("foo"              ), Result = Seq<(int, string?)>((1, "foo"), (2, null ), (3, null )) },
+                new { A = Seq(1, 2, 3), B = Seq("foo", "bar"       ), Result = Seq<(int, string?)>((1, "foo"), (2, "bar"), (3, null )) },
+                new { A = Seq(1, 2, 3), B = Seq("foo", "bar", "baz"), Result = Seq<(int, string?)>((1, "foo"), (2, "bar"), (3, "baz")) },
             }
             select new TestCaseData(e.A, e.B)
                 .Returns(e.Result);
diff --git a/MoreLinq/Extensions.g.cs b/MoreLinq/Extensions.g.cs
index 14d73e462..9e5b16a71 100644
--- a/MoreLinq/Extensions.g.cs
+++ b/MoreLinq/Extensions.g.cs
@@ -6837,7 +6837,7 @@ public static partial class ZipLongestExtension
         public static IEnumerable<TResult> ZipLongest<TFirst, TSecond, TResult>(
             this IEnumerable<TFirst> first,
             IEnumerable<TSecond> second,
-            Func<TFirst, TSecond, TResult> resultSelector)
+            Func<TFirst?, TSecond?, TResult> resultSelector)
             => MoreEnumerable.ZipLongest(first, second, resultSelector);
 
         /// <summary>
@@ -6883,7 +6883,7 @@ public static IEnumerable<TResult> ZipLongest<T1, T2, T3, TResult>(
             this IEnumerable<T1> first,
             IEnumerable<T2> second,
             IEnumerable<T3> third,
-            Func<T1, T2, T3, TResult> resultSelector)
+            Func<T1?, T2?, T3?, TResult> resultSelector)
             => MoreEnumerable.ZipLongest(first, second, third, resultSelector);
 
         /// <summary>
@@ -6933,7 +6933,7 @@ public static IEnumerable<TResult> ZipLongest<T1, T2, T3, T4, TResult>(
             IEnumerable<T2> second,
             IEnumerable<T3> third,
             IEnumerable<T4> fourth,
-            Func<T1, T2, T3, T4, TResult> resultSelector)
+            Func<T1?, T2?, T3?, T4?, TResult> resultSelector)
             => MoreEnumerable.ZipLongest(first, second, third, fourth, resultSelector);
 
     }
diff --git a/MoreLinq/ZipLongest.cs b/MoreLinq/ZipLongest.cs
index 5f93188d7..38e5fdefc 100644
--- a/MoreLinq/ZipLongest.cs
+++ b/MoreLinq/ZipLongest.cs
@@ -60,7 +60,7 @@ static partial class MoreEnumerable
         public static IEnumerable<TResult> ZipLongest<TFirst, TSecond, TResult>(
             this IEnumerable<TFirst> first,
             IEnumerable<TSecond> second,
-            Func<TFirst, TSecond, TResult> resultSelector)
+            Func<TFirst?, TSecond?, TResult> resultSelector)
         {
             if (first == null) throw new ArgumentNullException(nameof(first));
             if (second == null) throw new ArgumentNullException(nameof(second));
@@ -112,7 +112,7 @@ public static IEnumerable<TResult> ZipLongest<T1, T2, T3, TResult>(
             this IEnumerable<T1> first,
             IEnumerable<T2> second,
             IEnumerable<T3> third,
-            Func<T1, T2, T3, TResult> resultSelector)
+            Func<T1?, T2?, T3?, TResult> resultSelector)
         {
             if (first == null) throw new ArgumentNullException(nameof(first));
             if (second == null) throw new ArgumentNullException(nameof(second));
@@ -169,7 +169,7 @@ public static IEnumerable<TResult> ZipLongest<T1, T2, T3, T4, TResult>(
             IEnumerable<T2> second,
             IEnumerable<T3> third,
             IEnumerable<T4> fourth,
-            Func<T1, T2, T3, T4, TResult> resultSelector)
+            Func<T1?, T2?, T3?, T4?, TResult> resultSelector)
         {
             if (first == null) throw new ArgumentNullException(nameof(first));
             if (second == null) throw new ArgumentNullException(nameof(second));