diff --git a/src/FSharp.MongoDB.Bson/FSharp.MongoDB.Bson.fsproj b/src/FSharp.MongoDB.Bson/FSharp.MongoDB.Bson.fsproj
index 16bfccf..6a88462 100644
--- a/src/FSharp.MongoDB.Bson/FSharp.MongoDB.Bson.fsproj
+++ b/src/FSharp.MongoDB.Bson/FSharp.MongoDB.Bson.fsproj
@@ -4,6 +4,7 @@
     <TargetFramework>netstandard2.1</TargetFramework>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
     <Nullable>enable</Nullable>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/src/FSharp.MongoDB.Bson/Serialization/FSharpTypeHelpers.fs b/src/FSharp.MongoDB.Bson/Serialization/FSharpTypeHelpers.fs
index 6d8a4d0..6611087 100644
--- a/src/FSharp.MongoDB.Bson/Serialization/FSharpTypeHelpers.fs
+++ b/src/FSharp.MongoDB.Bson/Serialization/FSharpTypeHelpers.fs
@@ -34,17 +34,21 @@ module private Helpers =
     /// <summary>
     /// Returns <c>Some typ</c> when <c>typ</c> is a record type, and <c>None</c> otherwise.
     /// </summary>
-    let (|IsRecord|_|) typ =
-        let isRecord typ = typ <> null && FSharpType.IsRecord(typ, bindingFlags)
-        whenType isRecord typ
+    let (|IsRecord|_|) = function
+        | Null -> None
+        | NonNull typ ->
+            let isRecord typ = FSharpType.IsRecord(typ, bindingFlags)
+            whenType isRecord typ
 
     /// <summary>
     /// Returns <c>Some typ</c> when <c>typ</c> is a top-level union type or when it represents a
     /// particular union case, and <c>None</c> otherwise.
     /// </summary>
-    let (|IsUnion|_|) (typ:System.Type) =
-        let isUnion typ = typ <> null && FSharpType.IsUnion(typ, bindingFlags)
-        whenType isUnion typ
+    let (|IsUnion|_|) = function
+        | Null -> None
+        | NonNull typ ->
+            let isUnion typ = FSharpType.IsUnion(typ, bindingFlags)
+            whenType isUnion typ
 
     /// <summary>
     /// Returns true if <c>typ</c> is a generic type with defintion <c>'GenericType</c>.
diff --git a/src/FSharp.MongoDB.Bson/Serialization/FSharpValueSerializer.fs b/src/FSharp.MongoDB.Bson/Serialization/FSharpValueSerializer.fs
index 2aca1b5..a3e01ae 100644
--- a/src/FSharp.MongoDB.Bson/Serialization/FSharpValueSerializer.fs
+++ b/src/FSharp.MongoDB.Bson/Serialization/FSharpValueSerializer.fs
@@ -38,9 +38,10 @@ module FSharpValueSerializer =
         interface IBsonSerializationProvider with
 
             member __.GetSerializer typ =
-                let mkSerializer = function
-                | Some (typ:System.Type) -> System.Activator.CreateInstance typ :?> IBsonSerializer
-                | None -> null
+                let mkSerializer typ = 
+                    typ
+                    |> Option.map (fun typ -> System.Activator.CreateInstance typ :?> IBsonSerializer)
+                    |> Option.toObj
 
                 match typ with
                 | IsList typ -> Some (mkGenericUsingDef<FSharpListSerializer<_>> typ)
diff --git a/tests/FSharp.MongoDB.Bson.Tests/FSharp.MongoDB.Bson.Tests.fsproj b/tests/FSharp.MongoDB.Bson.Tests/FSharp.MongoDB.Bson.Tests.fsproj
index 2924ecd..aeca538 100644
--- a/tests/FSharp.MongoDB.Bson.Tests/FSharp.MongoDB.Bson.Tests.fsproj
+++ b/tests/FSharp.MongoDB.Bson.Tests/FSharp.MongoDB.Bson.Tests.fsproj
@@ -2,8 +2,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <TargetFramework>net9.0</TargetFramework>
-    <GenerateProgramFile>false</GenerateProgramFile>
     <Nullable>enable</Nullable>
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
   </PropertyGroup>
 
   <ItemGroup>
diff --git a/tests/FSharp.MongoDB.Bson.Tests/FSharpListSerializationTests.fs b/tests/FSharp.MongoDB.Bson.Tests/FSharpListSerializationTests.fs
index 0b0aa9e..aa4bca2 100644
--- a/tests/FSharp.MongoDB.Bson.Tests/FSharpListSerializationTests.fs
+++ b/tests/FSharp.MongoDB.Bson.Tests/FSharpListSerializationTests.fs
@@ -87,13 +87,17 @@ module FSharpListSerialization =
             let value = { MaybeStrings = [ Some "a"; None; Some "z" ] }
 
             let result = serialize value
-            let expected = BsonDocument("MaybeStrings", BsonArray [ "a"; null; "z" ])
+            let expected =
+                let values: (string|null) array = [| "a"; null; "z" |]
+                BsonDocument("MaybeStrings", BsonArray values)
 
             result |> should equal expected
 
         [<Test>]
         let ``test deserialize a list of optional strings``() =
-            let doc = BsonDocument("MaybeStrings", BsonArray [ "a"; null; "z" ])
+            let doc =
+                let values: (string | null) array = [| "a"; null; "z" |]
+                BsonDocument("MaybeStrings", BsonArray values)
 
             let result = deserialize doc typeof<Record>
             let expected = { MaybeStrings = [ Some "a"; None; Some "z" ] }
diff --git a/tests/FSharp.MongoDB.Bson.Tests/FSharpSetSerializationTests.fs b/tests/FSharp.MongoDB.Bson.Tests/FSharpSetSerializationTests.fs
index 46f1380..63aa4d4 100644
--- a/tests/FSharp.MongoDB.Bson.Tests/FSharpSetSerializationTests.fs
+++ b/tests/FSharp.MongoDB.Bson.Tests/FSharpSetSerializationTests.fs
@@ -87,13 +87,17 @@ module FSharpSetSerialization =
             let value = { MaybeStrings = Set.ofList [ Some "a"; None; Some "z" ] }
 
             let result = serialize value
-            let expected = BsonDocument("MaybeStrings", BsonArray (Set.ofList ["a"; null; "z"]))
+            let expected =
+                let values: (string|null) list = [ "a"; null; "z" ]
+                BsonDocument("MaybeStrings", BsonArray (Set.ofList values))
 
             result |> should equal expected
 
         [<Test>]
         let ``test deserialize a set of optional strings``() =
-            let doc = BsonDocument("MaybeStrings", BsonArray [ "a"; null; "z" ])
+            let doc =
+                let values: (string|null) list = [ "a"; null; "z" ]
+                BsonDocument("MaybeStrings", BsonArray values)
 
             let result = deserialize doc typeof<Record>
             let expected = { MaybeStrings = Set.ofList [ Some "a"; None; Some "z" ] }