You tried to modify the value of a variable that cannot
be modified. This error can be caused by the following:
 
  ■ Mistyping the name of the variable
  ■ Forgetting to place single quotation marks around a
    character constant
  ■ Using the name of a variable that is not local to the
    current function
  ■ Trying to use an enum constant to modify an enum
    or an integer
 
You can modify the value of a variable only while executing
a function in which it is defined.