diff --git a/index.html b/index.html
index 919bac7..e3afda7 100644
--- a/index.html
+++ b/index.html
@@ -80,6 +80,8 @@ input[type="text"] {
                 if (term !== null) {
                     const query = `type(empty, ${term}, ?tau)`;
                     app.ports.receiveConverted.send({ input: string, query });
+                } else {
+                    app.ports.receiveConverted.send({ input: string, query: null });
                 }
             });
 
diff --git a/src/Main.elm b/src/Main.elm
index 77c777d..a8e8af9 100644
--- a/src/Main.elm
+++ b/src/Main.elm
@@ -37,6 +37,11 @@ type ResultMode
     = Conclusion
     | Tree
 
+type DesugaringState
+    = Requested
+    | Succeeded String
+    | Failed
+
 resultModeEq : ResultMode -> ResultMode -> Bool
 resultModeEq rm1 rm2 =
     case (rm1, rm2) of
@@ -53,9 +58,8 @@ type alias Model =
       -- ^ The current tab (editor, redner rule editor, rendered)
     , input : String
       -- ^ The string the user has in the input box
-    , desugaredQuery : Maybe String
-      -- ^ The Bergamot query corresponding to input. May be Nothing while waiting for
-      --   Custom mode, which has user JS convert input to a query.
+    , desugaredQuery : DesugaringState
+      -- ^ The Bergamot query corresponding to input, if we're in custom mode.
     , inputModes : List (String, EditMode)
       -- ^ A List of input modes that can be used with the input box.
     , inputMode : Int
@@ -77,7 +81,7 @@ type Msg
     | SetTab Tab
     | SetInputMode Int
     | SetResultMode ResultMode
-    | SetDesugaredQuery { input: String, query: String }
+    | SetDesugaredQuery { input: String, query: Maybe String }
 
 decodeInputModes: Json.Decode.Value -> List (String, EditMode)
 decodeInputModes val =
@@ -102,10 +106,10 @@ decodeInputModes val =
 port convertInput : { mode : String, input : String } -> Cmd msg
 
 -- Invoked when user code finishes converting 'input' into a Bergamot query
-port receiveConverted : ({ input : String, query : String } -> msg) -> Sub msg
+port receiveConverted : ({ input : String, query : Maybe String } -> msg) -> Sub msg
 
-convertInputCmd : Int -> List (String, EditMode) -> String -> Cmd Msg
-convertInputCmd inputMode inputModes input =
+requestDesugaring : Int -> List (String, EditMode) -> String -> Cmd Msg
+requestDesugaring inputMode inputModes input =
     case getEditMode inputMode inputModes of
         Ok (Custom modeType) ->
             convertInput { mode = modeType, input = input }
@@ -120,13 +124,13 @@ init fs =
         ( { program = fs.rules
           , renderProgram = fs.renderRules
           , input = fs.input
-          , desugaredQuery = Nothing
+          , desugaredQuery = Failed
           , tab = Rendered
           , inputModes = inputModes
           , inputMode = 0
           , resultMode = Conclusion
           }
-        , convertInputCmd inputMode inputModes fs.input
+        , requestDesugaring inputMode inputModes fs.input
         )
 
 viewSection : String -> Html Msg -> Html Msg
@@ -236,7 +240,7 @@ getEditMode i l =
         ((_, editMode) :: xs) ->
             if i == 0 then Ok editMode else getEditMode (i - 1) xs
 
-proofGoal : EditMode -> String -> Maybe String -> Result Error (Term Metavariable)
+proofGoal : EditMode -> String -> DesugaringState -> Result Error (Term Metavariable)
 proofGoal editMode input desugaredQuery =
     if input == ""
     then Err Silent
@@ -248,11 +252,12 @@ proofGoal editMode input desugaredQuery =
                     Just query -> Ok query
             Custom _ ->
                 case desugaredQuery of
-                    Just querys ->
+                    Requested -> Err Silent
+                    Succeeded querys ->
                         case run topLevelTerm querys of
                             Nothing -> Err BadQuery
                             Just query -> Ok query
-                    Nothing -> Err Silent
+                    Failed -> Err BadObjectTerm
 
 progAndRenderProg : String -> String -> Result Error (RuleEnv, RuleEnv)
 progAndRenderProg progs renderProgs =
@@ -317,18 +322,21 @@ update msg m =
         SetRenderProgram prog -> ({ m | renderProgram = prog }, Cmd.none)
         SetInput input ->
             ( { m | input = input }
-            , convertInputCmd m.inputMode m.inputModes input
+            , requestDesugaring m.inputMode m.inputModes input
             )
         SetTab tab -> ({ m | tab = tab }, Cmd.none)
         SetInputMode mode ->
             ( { m | inputMode = mode }
-            , convertInputCmd mode m.inputModes m.input
+            , requestDesugaring mode m.inputModes m.input
             )
         SetResultMode mode -> ({ m | resultMode = mode }, Cmd.none)
         SetDesugaredQuery data ->
             ({ m | desugaredQuery =
                 if m.input == data.input
-                then Just data.query
+                then
+                    case data.query of
+                        Just q -> Succeeded q
+                        Nothing -> Failed
                 else m.desugaredQuery
              }
             , Cmd.none