This is a bug that occurs when you use JUnit 4.12 and PowerMock < 1.6.1. The problem is solved in PowerMock 1.6.1. Please update your dependencies accordingly
testCompile 'junit:junit:4.12',
'org.powermock:powermock-core:1.6.1',
'org.powermock:powermock-module-junit4:1.6.1',
'org.powermock:powermock-api-mockito:1.6.1'
If you cannot upgrade PowerMock then you can use JUnit 4.11.
testCompile 'junit:junit:4.11',
'org.powermock:powermock-core:1.5.6',
'org.powermock:powermock-module-junit4:1.5.6',
'org.powermock:powermock-api-mockito:1.5.6'
Could you please add further lines of the stacktrace, which uncover more details about the problem.