Error while installing .apk file on the android device

While installing the signed application on the android sony ericsson Xperia X10 following error is encountered:

pkg: /data/local/tmp/wai.apk
Failure [INSTALL_PARSE_FAILED_UNEXPECTED_EXCEPTION]

I have used the adb command to install it
C:\android-sdk-windows\tools>adb install wai.apk

Can anybody tell me the root cause of this error?