diff -r 11f920264454 Lib/json/encoder.py
--- a/Lib/json/encoder.py	Fri Mar 18 15:10:43 2016 +0100
+++ b/Lib/json/encoder.py	Wed Mar 23 10:50:51 2016 -0400
@@ -177,7 +177,7 @@
                 return JSONEncoder.default(self, o)
 
         """
-        raise TypeError(repr(o) + " is not JSON serializable")
+        raise TypeError(repr(o) + " of type:" + type(o) + " is not JSON serializable")
 
     def encode(self, o):
         """Return a JSON string representation of a Python data structure.