`System error, `, "bad id" ; what does it mean?

Hi everybody

I try to save an array of equations in a "mpl file", and then read file and array by command read mpl file. But system error "bad id" occurs. Previously I saved this mpl file by "save" and read array by "read" command. I ran this code several times without any problem, suddenly error "bad id" occurred. When I save mpl file by the code edit region and then read it by "read" in the same or  a new worksheet, the error doesn't happen!
 What is wrong?
"Id" refers to the array index?

Thanks.
Maple 18 and Windows 10