In general when using Increment or Decrement it’s not suggested you use the Store method, but instead start with either Increment or Decrement. Here is some additional information:
"If you want to set the value of the counter with add/set/replace, the objects data must be the ascii representation of the value and not the byte values of a 64 bit integer. "
- https://code.google.com/p/memcached/wiki/BinaryProtocolRevamped#Increment,_Decrement
- http://docs.couchbase.com/couchbase-devguide-2.0/#incrementing-and-decrementing
The solution/answer is to always use increment/decrement operations when you want use a key for counting (which is a best practice).
-Jeff