RooFit getError)= returns incorrect value

 Hi, rooters,

After fitting with RooFit, I did:
  frac.Print();

the result is
  RooRealVar::0xbab1c08::frac: 0.73801 +/- (-0.0472, 0.0466) L(0 - 1)

Note that the error value is not unique. Is it common and determined by the fitting mechanism OR is it some error message caused by inappropriate initialization of the parameters?

If it is the second case, how to avoid it? If it is the first case, how to get the error values? I did  frac.getError()
but it returned 0.

Thanks in advance for any help!

Cheers!
Xianguo

-- 
Take flight into the sky, beyond the moon, beyond my mind.

Xianguo LU
Physics Department, Peking University
Beijing, China
Tel: 0086-10-62753888(o)