diff --git a/Imms/ExtraFunctional/ExtraFunctional/Option.fs b/Imms/ExtraFunctional/ExtraFunctional/Option.fs
index fe4b22a..3af9160 100644
--- a/Imms/ExtraFunctional/ExtraFunctional/Option.fs
+++ b/Imms/ExtraFunctional/ExtraFunctional/Option.fs
@@ -2,7 +2,7 @@
 module Option = 
     let orValue (v : 'v) = function Some u -> u | None -> v
     let orMaybe (v : 'v option) = function Some u -> Some u | None -> v
-    let asNull = Option.toObj
+    let asNull (v : 'v option) = if v.IsNone then null else v.Value
     let cast<'a,'b> (opt : 'a option) : 'b option = opt |> Option.map (fun a -> a :> obj :?> 'b)
     
 [<AutoOpen>]
diff --git a/Imms/Imms.FSharp/Imms.FSharp.fsproj b/Imms/Imms.FSharp/Imms.FSharp.fsproj
index 40db670..c812480 100644
--- a/Imms/Imms.FSharp/Imms.FSharp.fsproj
+++ b/Imms/Imms.FSharp/Imms.FSharp.fsproj
@@ -85,10 +85,6 @@
     <Content Include="packages.config" />
   </ItemGroup>
   <ItemGroup>
-    <Reference Include="FSharp.Core">
-      <HintPath>..\..\..\packages\FSharp.Core.4.0.0.1\lib\net40\FSharp.Core.dll</HintPath>
-      <Private>True</Private>
-    </Reference>
     <Reference Include="mscorlib" />
     <Reference Include="System" />
     <Reference Include="System.Core" />
diff --git a/Imms/Imms.Tests.Integrity/App.config b/Imms/Imms.Tests.Integrity/App.config
index 3fd002c..e4902b5 100644
--- a/Imms/Imms.Tests.Integrity/App.config
+++ b/Imms/Imms.Tests.Integrity/App.config
@@ -7,7 +7,7 @@
     <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
       <dependentAssembly>
         <assemblyIdentity name="FSharp.Core" publicKeyToken="b03f5f7f11d50a3a" culture="neutral" />
-        <bindingRedirect oldVersion="2.0.0.0" newVersion="4.3.0.0" />
+        <bindingRedirect oldVersion="0.0.0.0-4.3.0.0" newVersion="4.3.0.0" />
         <bindingRedirect oldVersion="2.3.5.0" newVersion="4.3.0.0" />
         <bindingRedirect oldVersion="4.0.0.0" newVersion="4.3.0.0" />
       </dependentAssembly>
diff --git a/Imms/Imms.Tests.Integrity/Imms.Tests.Integrity.fsproj b/Imms/Imms.Tests.Integrity/Imms.Tests.Integrity.fsproj
index aa51422..047f7c0 100644
--- a/Imms/Imms.Tests.Integrity/Imms.Tests.Integrity.fsproj
+++ b/Imms/Imms.Tests.Integrity/Imms.Tests.Integrity.fsproj
@@ -1,5 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="12.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="..\packages\xunit.runner.visualstudio.2.1.0\build\net20\xunit.runner.visualstudio.props" Condition="Exists('..\packages\xunit.runner.visualstudio.2.1.0\build\net20\xunit.runner.visualstudio.props')" />
   <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
   <PropertyGroup>
     <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
@@ -16,6 +17,8 @@
     <TargetFrameworkProfile />
     <SolutionDir Condition="$(SolutionDir) == '' Or $(SolutionDir) == '*Undefined*'">..\</SolutionDir>
     <RestorePackages>true</RestorePackages>
+    <NuGetPackageImportStamp>
+    </NuGetPackageImportStamp>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
     <DebugSymbols>true</DebugSymbols>
@@ -42,8 +45,12 @@
     <Prefer32Bit>true</Prefer32Bit>
   </PropertyGroup>
   <ItemGroup>
-    <Reference Include="FSharp.Core">
-      <HintPath>..\..\..\packages\FSharp.Core.4.0.0.1\lib\net40\FSharp.Core.dll</HintPath>
+    <Reference Include="FsCheck">
+      <HintPath>..\packages\FsCheck.2.6.0\lib\net45\FsCheck.dll</HintPath>
+      <Private>True</Private>
+    </Reference>
+    <Reference Include="FsCheck.Xunit">
+      <HintPath>..\packages\FsCheck.Xunit.2.6.0\lib\net45\FsCheck.Xunit.dll</HintPath>
       <Private>True</Private>
     </Reference>
     <Reference Include="mscorlib" />
@@ -54,6 +61,18 @@
     </Reference>
     <Reference Include="System.Core" />
     <Reference Include="System.Numerics" />
+    <Reference Include="xunit.abstractions">
+      <HintPath>..\packages\xunit.abstractions.2.0.0\lib\net35\xunit.abstractions.dll</HintPath>
+      <Private>True</Private>
+    </Reference>
+    <Reference Include="xunit.core">
+      <HintPath>..\packages\xunit.extensibility.core.2.1.0\lib\portable-net45+win8+wp8+wpa81\xunit.core.dll</HintPath>
+      <Private>True</Private>
+    </Reference>
+    <Reference Include="xunit.execution.desktop">
+      <HintPath>..\packages\xunit.extensibility.execution.2.1.0\lib\net45\xunit.execution.desktop.dll</HintPath>
+      <Private>True</Private>
+    </Reference>
   </ItemGroup>
   <ItemGroup>
     <Compile Include="Definitions.fs" />
@@ -112,6 +131,7 @@
       <ErrorText>This project references NuGet package(s) that are missing on this computer. Enable NuGet Package Restore to download them.  For more information, see http://go.microsoft.com/fwlink/?LinkID=322105. The missing file is {0}.</ErrorText>
     </PropertyGroup>
     <Error Condition="!Exists('$(SolutionDir)\.nuget\NuGet.targets')" Text="$([System.String]::Format('$(ErrorText)', '$(SolutionDir)\.nuget\NuGet.targets'))" />
+    <Error Condition="!Exists('..\packages\xunit.runner.visualstudio.2.1.0\build\net20\xunit.runner.visualstudio.props')" Text="$([System.String]::Format('$(ErrorText)', '..\packages\xunit.runner.visualstudio.2.1.0\build\net20\xunit.runner.visualstudio.props'))" />
   </Target>
   <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
        Other similar extension points exist, see Microsoft.Common.targets.
diff --git a/Imms/Imms.Tests.Integrity/MapTests.fs b/Imms/Imms.Tests.Integrity/MapTests.fs
index 620fb71..11c756d 100644
--- a/Imms/Imms.Tests.Integrity/MapTests.fs
+++ b/Imms/Imms.Tests.Integrity/MapTests.fs
@@ -234,13 +234,90 @@ type MapTests<'e when 'e : comparison>(items : 'e array, ?seed : int) =
     member x.Find iters = create_test iters "Find" (x.find iters >> toList1 >> List.cast)
     member x.Gen_disjoint iters = create_test iters "GenDisjoint" (x.gen_equal_keys_dif_values iters >> toList1 >> List.cast)
 
+open Xunit
+open FsCheck
+open FsCheck.Xunit
 
+module MapTestsFsCheck =
+    type [<RequireQualifiedAccess>] MapAction<'k, 'v> =
+        | SetKey of 'k * 'v
+        | RemoveRandom
 
+    /// Keeps a reference to all persistent collections returned after
+    /// performing an action, and after they are all applied, checks
+    /// that they equal what we would get from FSharp.Core.Map
+    let inline eqMapsAfterSteps
+        (fsMap:Map<'k, 'v>)
+        (testMap:'m)
+        (actions: MapAction<'k, 'v>[])
+        (setKey: 'k->'v->'m->'m when 'k: comparison and 'v: comparison)
+        (removeKey: 'k->'m->'m when 'k: comparison and 'v: comparison)
+        (eqMap: 'm->Map<'k, 'v>->bool) =
 
+        let applyAction fsMap testMap action =
+            match action with
+            | MapAction.SetKey(k, v) ->
+                (Map.add k v fsMap, setKey k v testMap)
+            | MapAction.RemoveRandom when Map.isEmpty fsMap ->
+                (fsMap, testMap)
+            | MapAction.RemoveRandom ->
+                let idx = Gen.choose(0, fsMap.Count - 1).Sample(0, 1).Head
+                let ary = Map.toArray fsMap
+                let key = fst ary.[idx]
+                (Map.remove key fsMap, removeKey key testMap)
 
+        let (fsMaps, testMaps) =
+            Array.fold
+                (fun acc action ->
+                    match acc with
+                    | (fsMap::fsMaps, testMap::testMaps) ->
+                        let (newF, newT) = applyAction fsMap testMap action
+                        (newF::fsMap::fsMaps, newT::testMap::testMaps)
+                    | _ -> failwith "Logic error")
+                ([fsMap], [testMap])
+                actions
 
+        List.forall2
+            eqMap
+            testMaps
+            fsMaps
 
-            
+    let immMap2FsMap (m:ImmMap<_,_>) =
+        Seq.fold
+            (fun acc (KeyValue(k,v)) -> Map.add k v acc)
+            (Map.empty)
+            (m.ToArray())
 
+    [<Property>]
+    let immMapsEqualFsMapsAfterSteps (startFsMap:Map<int, int>) (actions: MapAction<int, int>[]) =
+        let testMap = ImmMap.ToImmMap startFsMap
+        let eqMap (m:ImmMap<_,_>) (fsMap:Map<_,_>) =
+            immMap2FsMap m = fsMap
 
+        eqMapsAfterSteps 
+            startFsMap
+            testMap
+            actions
+            (ImmMap.set)
+            (ImmMap.remove)
+            eqMap
 
+    let sortedMap2FsMap (m:ImmSortedMap<_,_>) =
+        Seq.fold
+            (fun acc (KeyValue(k,v)) -> Map.add k v acc)
+            (Map.empty)
+            (m.ToArray())
+
+    [<Property>]
+    let immSortedMapsEqualFsMapsAfterSteps (startFsMap:Map<int, int>) (actions: MapAction<int, int>[]) =
+        let testMap = ImmSortedMap.ToImmSortedMap startFsMap
+        let eqMap (m:ImmSortedMap<_,_>) (fsMap:Map<_,_>) =
+            sortedMap2FsMap m = fsMap
+
+        eqMapsAfterSteps 
+            startFsMap
+            testMap
+            actions
+            (ImmSortedMap.set)
+            (ImmSortedMap.remove)
+            eqMap
\ No newline at end of file
diff --git a/Imms/Imms.Tests.Integrity/packages.config b/Imms/Imms.Tests.Integrity/packages.config
index fdf5fe2..30f0eb5 100644
--- a/Imms/Imms.Tests.Integrity/packages.config
+++ b/Imms/Imms.Tests.Integrity/packages.config
@@ -1,5 +1,11 @@
 <?xml version="1.0" encoding="utf-8"?>
 <packages>
+  <package id="FsCheck" version="2.6.0" targetFramework="net451" />
+  <package id="FsCheck.Xunit" version="2.6.0" targetFramework="net451" />
   <package id="FSharp.Core" version="4.0.0.1" targetFramework="net451" />
   <package id="Microsoft.Bcl.Immutable" version="1.0.34" targetFramework="net45" />
+  <package id="xunit.abstractions" version="2.0.0" targetFramework="net451" />
+  <package id="xunit.extensibility.core" version="2.1.0" targetFramework="net451" />
+  <package id="xunit.extensibility.execution" version="2.1.0" targetFramework="net451" />
+  <package id="xunit.runner.visualstudio" version="2.1.0" targetFramework="net451" />
 </packages>
\ No newline at end of file