diff --git a/src/Common.fs b/src/Common.fs
index bc1c7f96..a4835e8c 100644
--- a/src/Common.fs
+++ b/src/Common.fs
@@ -6,7 +6,7 @@ module Common =
     open Microsoft.FSharp.Reflection
 
     let isUnionCase x =
-        FSharpType.IsUnion(<@ x @>.Type) |> not && (box x) = null
+        FSharpType.IsUnion(<@ x @>.Type) |> not && isNull(box x)
         || FSharpType.IsUnion(x.GetType())
 
     /// <summary>
diff --git a/src/FsUnit.Xunit/CustomMatchers.fs b/src/FsUnit.Xunit/CustomMatchers.fs
index 760cfbe3..926f90eb 100644
--- a/src/FsUnit.Xunit/CustomMatchers.fs
+++ b/src/FsUnit.Xunit/CustomMatchers.fs
@@ -40,14 +40,14 @@ let equalWithin (tolerance: obj) (expected: obj) =
     let matches(actual: obj) =
         let parseValue(v: string) =
             match Double.TryParse(v, NumberStyles.Any, CultureInfo("en-US")) with
-            | true, x -> Some(x)
-            | false, _ -> None
+            | true, x -> ValueSome x
+            | false, _ -> ValueNone
 
         let actual = string actual |> parseValue
         let expect = string expected |> parseValue
         let tol = string tolerance |> parseValue
 
-        if [| actual; expect; tol |] |> Array.contains None |> not then
+        if [| actual; expect; tol |] |> Array.contains ValueNone |> not then
             abs(actual.Value - expect.Value) <= tol.Value
         else
             false