Powermock 1.6.3 uses javassist 3.15.2-GA which does not support certain types. Using 3.18.2-GA javassist worked for me. You may want to override dependency in your project.
<dependency>
<groupId>org.javassist</groupId>
<artifactId>javassist</artifactId>
<version>3.18.2-GA</version>
</dependency>
You may face another problem for which the solution lies here Mockito + PowerMock LinkageError while mocking system class
Hope this helps.