Question about DLL using MFC

Hi! I am new to the DLL world. I have created a simple class declared as
following:

class __declspec(dllexport) myapi
{
public:
 // status
 bool error; // last status
 char *errmsg; // if error is true, this is the reason

 // default constructor and destructor
 myapi();
 virtual ~myapi();

private:
 void setErrmsg(const char *str);

};

and the setErrmsg() is implemented as:
void myapi::setErrmsg(const char *str)
{
 AFX_MANAGE_STATE(AfxGetStaticModuleState());

 int len;
 len = strlen(str);
 if(len != 0) {
  if(errmsg != NULL) {
   delete [] errmsg;
   errmsg = NULL;
  }
  errmsg = new char[len];
  strcpy(errmsg, str);
 } else {
  if(errmsg != NULL) {
   delete [] errmsg;
   errmsg = NULL;
  }
 }
}

in the destructor, I just wrote:
  if(errmsg != NULL) {
   delete [] errmsg;
   errmsg = NULL;
  }
to free up the memory.

Then I tried this DLL in VS2005, by:
mydll *obj;
obj = new mydll();

and I can get the errmsg by
String ^str = gcnew String(obj->errmsg);
I also tried:
       String ^str = String::Empty;
       int len;
       len = (int) strlen(obj->errmsg);
       if(len > 0) {
        int i;
        for(i = 0; i < len; ++i) {
         str = str->PadRight(i+1, obj->errmsg[i]);
        }
       }
to copy all the bytes from DLL to my application.

The problem occurs when I tried:
delete obj;

VS2005will issue a memory check error when the destructor of myapl executed
to the delete [] errmsg; statement.

Can anyone tell me how to avoid this problem? Any hint is welcome!

Thank you and best regards,

Tim