Fake deploy package issue

Question

I'm new at FAKE and I'm trying to use fake as continuous delivery, but I'm faced with a problem with deploying my .nupkg files. Here is the code of my DeployPackage target

Target "DeployPackages" (fun _ -> 
deployProjects
    |> Seq.iter (fun projName ->
        deploymentMachines
            |> Seq.iter (fun machineUrl -> 
                let package = Directory.GetFiles(deployDir, projName + "*.nupkg").FirstOrDefault()

                if package = String.Empty 
                    then failwith "No packages was found. You should get green build before deployment."
                else
                    package |> deployToMachine(machineUrl)
            )
    )
)

and in TeamCity build log I see error:

"Finished Target: CreateDeploymentPackage Starting Target: DeployPackages (==> CreateDeploymentPackage, CreateDeploymentPackage)
Target: DeployPackages ..\Build\output\deploy\MySitev2.65.nupkg"

AND then the next error message "Newtonsoft.Json.JsonSerializationException:

No union type found with the name 'Message'. Path 'case', line 2, position 20.
at Newtonsoft.Json.Converters.DiscriminatedUnionConverter.ReadJson(JsonReader reader, Type objectType, Object existingValue, JsonSerializer serializer) at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.DeserializeConvertable(JsonConverter converter, JsonReader reader, Type objectType, Object existingValue)
[15:35:09][Step 2/2] at Newtonsoft.Json.Serialization.JsonSerializerInternalReader.Deserialize(JsonReader reader, Type objectType, Boolean checkAdditionalContent)
[15:35:09][Step 2/2] at Newtonsoft.Json.JsonSerializer.DeserializeInternal(JsonReader reader, Type objectType)
at Newtonsoft.Json.JsonConvert.DeserializeObject(String value, Type type, JsonSerializerSettings settings)
at Newtonsoft.Json.JsonConvert.DeserializeObject[T](String value, JsonSerializerSettings settings)
at Fake.FakeDeployAgentHelper.processResponse(Byte[] response) in D:\code\fake\src\app\Fake.Deploy.Lib\FakeDeployAgentHelper.fs:line 120"

Whats wrong with my target?