diff --git a/src/sitelets/WebSharper.Web/ClientSideJson.fs b/src/sitelets/WebSharper.Web/ClientSideJson.fs
index ba943103d..722be2fd2 100644
--- a/src/sitelets/WebSharper.Web/ClientSideJson.fs
+++ b/src/sitelets/WebSharper.Web/ClientSideJson.fs
@@ -79,7 +79,7 @@ module Provider =
     let EncodeUnion (_: obj) (discr: string) (cases: (string * (string * string * (unit -> obj -> obj) * OptionalFieldKind)[])[]) : (unit -> 'T -> obj) =
         ()
         fun () x ->
-            if JS.TypeOf x ===. JS.Object then
+            if JS.TypeOf x ===. JS.Object && x !=. null then
                 let o = New []
                 let tag = x?("$")
                 let tagName, fields = cases.[tag]
diff --git a/tests/WebSharper.Sitelets.Tests/Json.fs b/tests/WebSharper.Sitelets.Tests/Json.fs
index 8fe4e6a3a..c0e4bacf8 100644
--- a/tests/WebSharper.Sitelets.Tests/Json.fs
+++ b/tests/WebSharper.Sitelets.Tests/Json.fs
@@ -76,6 +76,7 @@ module Json =
             | [<Constant "foo">] Foo
             | [<Constant "bar">] Bar
             | [<Constant 12>] Twelve
+            | [<Constant null>] Null
 
         type GenericUnion() =
 
diff --git a/tests/WebSharper.Web.Tests/ClientSideJson.fs b/tests/WebSharper.Web.Tests/ClientSideJson.fs
index 34c875c4c..03a67fbb5 100644
--- a/tests/WebSharper.Web.Tests/ClientSideJson.fs
+++ b/tests/WebSharper.Web.Tests/ClientSideJson.fs
@@ -286,12 +286,14 @@ module ClientSideJson =
                 equal (Json.Serialize Foo |> Json.Parse |> unbox) "foo"
                 equal (Json.Serialize Bar |> Json.Parse |> unbox) "bar"
                 equal (Json.Serialize Twelve |> Json.Parse |> unbox) 12
+                equal (Json.Serialize Null |> Json.Parse |> unbox) null
             }
 
             Test "deserialize union with constants" {
                 equal (Json.Deserialize (Json.Stringify "foo")) Foo
                 equal (Json.Deserialize (Json.Stringify "bar")) Bar
                 equal (Json.Deserialize (Json.Stringify 12)) Twelve
+                equal (Json.Deserialize (Json.Stringify null)) Null
             }
 
             Test "serialize System.DateTime" {
@@ -462,6 +464,7 @@ module ClientSideJson =
                 equalAsync (f Foo) Foo
                 equalAsync (f Bar) Bar
                 equalAsync (f Twelve) Twelve
+                equalAsync (f Null) Null
             }
 
             let now = System.DateTime.Now