Pointers, typedef's and const's

The code below can't be compiled:

  typedef char* POINTER;
  const POINTER ptr;
  ptr++;

The compiler (Sun C 5.8 Patch 121015-04 2007/01/10) complains:

  "test.c", line ...: operand must be modifiable lvalue: op "++"

It looks like the "const POINTER" has been compiled to "char* const".
A question is - why?

Best wishes,
Alex