Natbib package error

Dear All,

I used MikTeX version 2.7 and natbib package version number 8.3. I attached
my code below:

\documentclass{article}
\usepackage{url}
\usepackage{natbib}%

\begin{document}

This is for test \citet{HofnerPathPlanningCleaning95}

\begin{thebibliography}{99}
\bibitem{HofnerPathPlanningCleaning95}
Hofner, C., Schmidt, G.: Path planning and guidance techniques for an
  autonomous mobile cleaning robot. Robot. Auton. Syst. {\bf 14}, 199--212
(1995)

\bibitem{AcarPathPlanningRoboticDemining03}
Acar, E.U.: Path planning for robotic demining: robust sensor-based coverage
  of unstructured environments and probabilistic methods.
  Int. J. Rob. Res. {\bf 22}, 441--466 (2003)
\end{thebibliography}

%%%%%%%%%

If I run this file, then I got the below mentinoed error:

************

! Package natbib Error: Bibliography not compatible with author-year
citations.
(natbib)                Press <return> to continue in numerical citation
style.
See the natbib package documentation for explanation.
Type  H <return>  for immediate help.
\NAT@force@numbers{}\NAT@force@numbers
?
************

But the same file working nice with MikTeX version 2.4 with natbib package
version number 7.1.

Please suggest in this regard.

Thanks,
Saravanan,M.
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