Add the following test to jsontests.cpp class BinData80 : public Base { virtual BSONObj bson() const { char z[ 3 ]; z[ 0 ] = 'a'; z[ 1 ] = 'b'; z[ 2 ] = 'c'; BSONObjBuilder b; b.appendBinData( "a", 3, bdtCustom, z ); return b.obj(); } virtual string json() const { return "{ \"a\" : { \"$binary\" : \"YWJj\", \"$type\" : \"80\" } }"; } }; and you get the error JsonTests::FromJsonTests::BinData80 std::exception: code FailedToParse: FailedToParse: Argument of $type in $bindata object must be a hex string representation of a single byte: offset:50 in test JsonTests::FromJsonTests::BinData80