diff --git a/MoreLinq.Test/FillBackwardTest.cs b/MoreLinq.Test/FillBackwardTest.cs
index 0e5ccfc78..337cf2300 100644
--- a/MoreLinq.Test/FillBackwardTest.cs
+++ b/MoreLinq.Test/FillBackwardTest.cs
@@ -82,7 +82,7 @@ public void FillBackwardWithFillSelector()
 
             var result =
                 xs.Select(x => new { X = x, Y = x })
-                  .FillBackward(e => e.X == 0, (nm, m) => new { m.X, nm.Y });
+                  .FillBackward(e => e.X == 0, (m, nm) => new { m.X, nm.Y });
 
             Assert.That(result, Is.EquivalentTo(new[]
             {
diff --git a/MoreLinq.Test/FillForwardTest.cs b/MoreLinq.Test/FillForwardTest.cs
index 1493c67f5..a83e5277b 100644
--- a/MoreLinq.Test/FillForwardTest.cs
+++ b/MoreLinq.Test/FillForwardTest.cs
@@ -99,8 +99,8 @@ select line.Trim() into line
                     Value     = int.Parse(val),
                 });
 
-            data = data.FillForward(e => e.Continent == "-", (f, e) => new { f.Continent, e.Country, e.City, e.Value })
-                        .FillForward(e => e.Country   == "-", (f, e) => new { e.Continent, f.Country, e.City, e.Value });
+            data = data.FillForward(e => e.Continent == "-", (e, f) => new { f.Continent, e.Country, e.City, e.Value })
+                        .FillForward(e => e.Country   == "-", (e, f) => new { e.Continent, f.Country, e.City, e.Value });
 
 
             Assert.That(data, Is.EquivalentTo(new[]
diff --git a/MoreLinq/FillBackward.cs b/MoreLinq/FillBackward.cs
index 0f4076557..addf1f079 100644
--- a/MoreLinq/FillBackward.cs
+++ b/MoreLinq/FillBackward.cs
@@ -119,7 +119,7 @@ static IEnumerable<T> FillBackwardImpl<T>(IEnumerable<T> source, Func<T, bool> p
                         foreach (var blank in blanks)
                         {
                             yield return fillSelector != null
-                                       ? fillSelector(item, blank)
+                                       ? fillSelector(blank, item)
                                        : item;
                         }
 
diff --git a/MoreLinq/FillForward.cs b/MoreLinq/FillForward.cs
index 7315fca8c..1200afa6c 100644
--- a/MoreLinq/FillForward.cs
+++ b/MoreLinq/FillForward.cs
@@ -114,7 +114,7 @@ static IEnumerable<T> FillForwardImpl<T>(IEnumerable<T> source, Func<T, bool> pr
                 {
                     yield return seeded
                                ? fillSelector != null
-                                 ? fillSelector(seed, item)
+                                 ? fillSelector(item, seed)
                                  : seed
                                : item;
                 }