why Decode error come out during replay log data after expanding message type

[DinnerHowe]

for instance, example_t.lcm is:

package example;

struct example_t
{
    double x; 
}

then i record a lcm log named "example_log_data" undering such msg

after that expand this msg to

package example;

struct example_t
{
    double x; 
    double y;
}

and i replay "example_log_data" again undering new example_t.lcm msg type

Decode error come out

could anyone show me why this happened?

thx advance