Help with function "fitdistr" in "MASS"

Hi, R users:

I want to fit my data into a normal distribution by using the command
"fitdistr" in "MASS".
I changed my data class from "ts" to "numeric" by

class(mydata)="numeric"

but after using "fitdistr", I got the result below

fitdistr(mydata,"normal")

  mean    sd 
  NA     NA  
 (NA)   (NA) 

the help doc of "fitdistr" does not mention anything about that, thus I need
your help.

Thank you in advanced,
Saji from Shanghai