Google App Engine

Uncaught exception from servlet
java.lang.NoClassDefFoundError: org/apache/tools/ant/UnsupportedElementException

Caused by: java.lang.ClassNotFoundException: org.apache.tools.ant.UnsupportedElementException
at com.google.appengine.runtime.Request.process-d970675895461cb2(Request.java)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
... 44 more

エラーが出た。治らない。