diff --git a/src/Data/Textile/Query.elm b/src/Data/Textile/Query.elm
index dbb397e1c..3a31853b5 100644
--- a/src/Data/Textile/Query.elm
+++ b/src/Data/Textile/Query.elm
@@ -110,12 +110,6 @@ buildApiQuery clientUrl query =
 
 decode : Decoder Query
 decode =
-    let
-        -- Note: Using Json.Decode.Extra's optionalField here because we want
-        -- a failure when a Maybe decoded field value is invalid
-        strictOptional field decoder =
-            DE.andMap (DE.optionalField field decoder)
-    in
     Decode.succeed Query
         |> strictOptional "airTransportRatio" Split.decodeFloat
         |> strictOptional "business" Economics.decodeBusiness
@@ -143,13 +137,20 @@ decode =
         |> strictOptional "yarnSize" Unit.decodeYarnSize
 
 
+strictOptional : String -> Decoder a -> Decoder (Maybe a -> b) -> Decoder b
+strictOptional field decoder =
+    -- Note: Using Json.Decode.Extra's optionalField here because we want
+    -- a failure when a Maybe decoded field value is invalid
+    DE.andMap (DE.optionalField field decoder)
+
+
 decodeMaterialQuery : Decoder MaterialQuery
 decodeMaterialQuery =
     Decode.succeed MaterialQuery
-        |> Pipe.optional "country" (Decode.maybe Country.decodeCode) Nothing
+        |> strictOptional "country" Country.decodeCode
         |> Pipe.required "id" (Decode.map Material.Id Decode.string)
         |> Pipe.required "share" Split.decodeFloat
-        |> Pipe.optional "spinning" (Decode.maybe Spinning.decode) Nothing
+        |> strictOptional "spinning" Spinning.decode
 
 
 encode : Query -> Encode.Value
diff --git a/tests/Server/RouteTest.elm b/tests/Server/RouteTest.elm
index 950d06cd8..a65571e30 100644
--- a/tests/Server/RouteTest.elm
+++ b/tests/Server/RouteTest.elm
@@ -15,6 +15,7 @@ import Data.Textile.Step.Label as Label
 import Data.Unit as Unit
 import Dict exposing (Dict)
 import Expect
+import Html.Attributes exposing (disabled)
 import Json.Encode as Encode
 import Server.Route as Route
 import Static.Db as StaticDb
@@ -233,7 +234,33 @@ textileEndpoints db =
                     }
                 )
             |> expectTextileSingleErrorContains "physicalDurability"
-            |> asTest "should reject invalid POST body"
+            |> asTest "should reject invalid physicalDurability"
+        , "/textile/simulator"
+            |> testEndpoint db
+                "POST"
+                (Query.encode
+                    { tShirtCotonFrance
+                        | countrySpinning = Just (Country.Code "invalid")
+                    }
+                )
+            |> expectTextileSingleErrorContains "materials"
+            |> asTest "should reject invalid spinning country"
+        , "/textile/simulator"
+            |> testEndpoint db
+                "POST"
+                (Query.encode
+                    { tShirtCotonFrance
+                        | materials =
+                            [ { country = Just (Country.Code "invalid")
+                              , id = Material.Id "ei-coton"
+                              , share = Split.full
+                              , spinning = Nothing
+                              }
+                            ]
+                    }
+                )
+            |> expectTextileSingleErrorContains "materials"
+            |> asTest "should reject invalid materials country"
         ]
     , describe "materials param checks"
         [ let