diff --git a/src/Tests/Helpers.fs b/src/Tests/Helpers.fs
index 05fcdd911..381fa6c4b 100644
--- a/src/Tests/Helpers.fs
+++ b/src/Tests/Helpers.fs
@@ -20,7 +20,7 @@ let createSimpleDeployment parameters =
 
 let convertTo<'T> = Serialization.toJson >> Serialization.ofJson<'T>
 
-let farmerToMs<'T when 'T: null> (serializationSettings: Newtonsoft.Json.JsonSerializerSettings) data =
+let farmerToMs<'T when 'T: null and 'T: not struct> (serializationSettings: Newtonsoft.Json.JsonSerializerSettings) data =
     data
     |> Serialization.toJson
     |> fun json -> SafeJsonConvert.DeserializeObject<'T>(json, serializationSettings)