android - The output jar is empty. Did you specify the proper '-keep' options? -

android - The output jar is empty. Did you specify the proper '-keep' options? -

i trying build release apk file ant , m getting in console

d:\androidsdk\android-sdk_r18-windows\android-sdk-windows\tools\ant\build.xml:595: next error occurred while executing line:d:\androidsdk\android-sdk_r18windows\android-sdk-windows\tools\ant\build.xml:864: output jar empty. did specify proper '-keep' options?

what error , how can resolve it..m using latest sdk(api-17) revision 21.1 java_home,ant_home set

for library projects, needed add together -dontshrink alternative proguard config file - proguard shrinking classes.

android android-build