diff --git a/Implab/Diagnostics/Extensions.cs b/Implab/Diagnostics/Extensions.cs
--- a/Implab/Diagnostics/Extensions.cs
+++ b/Implab/Diagnostics/Extensions.cs
@@ -19,7 +19,7 @@
                 },
                 reason => {
                     TraceContext.Instance.EnterLogicalOperation(op,true);
-                    TraceLog.TraceInformation("promise cancelled {0}", reason.Message);
+                    TraceLog.TraceInformation("promise cancelled {0}", reason == null ? "<no-reason>" : reason.Message);
                     TraceLog.EndLogicalOperation();
                     TraceContext.Instance.Leave();
                 }
diff --git a/Implab/JSON/JSONParser.cs b/Implab/JSON/JSONParser.cs
--- a/Implab/JSON/JSONParser.cs
+++ b/Implab/JSON/JSONParser.cs
@@ -46,10 +46,11 @@ namespace Implab.JSON {
         static readonly DFAStateDescriptior[] _arrayDFA;
 
         static JSONParser() {
-            var jsonExpression = Token.New(JsonTokenType.BeginObject, JsonTokenType.BeginArray).Tag(0);
+
 
             var valueExpression = Token.New(JsonTokenType.BeginArray, JsonTokenType.BeginObject, JsonTokenType.Literal, JsonTokenType.Number, JsonTokenType.String);
             var memberExpression = Token.New(JsonTokenType.String).Cat(Token.New(JsonTokenType.NameSeparator)).Cat(valueExpression);
+
             var objectExpression = memberExpression
                 .Cat(
                     Token.New(JsonTokenType.ValueSeparator)
@@ -69,6 +70,8 @@ namespace Implab.JSON {
                 .Cat(Token.New(JsonTokenType.EndArray))
                 .Tag(0);
 
+            var jsonExpression = valueExpression.Tag(0);
+
             _jsonDFA = BuildDFA(jsonExpression).States;
             _objectDFA = BuildDFA(objectExpression).States;
             _arrayDFA = BuildDFA(arrayExpression).States;