CompletableFuture cannot be returned by getPassword() getPassword() should return byte[] *** If you're unsure why you're getting above error read on. Due to the nature of the syntax above problem might occur because: 1. This exception *might* occur in wrongly written multi-threaded tests. Please refer to Mockito FAQ on limitations of concurrency testing. 2. A spy is stubbed using when(spy.foo()).then() syntax. It is safer to stub spies - - with doReturn|Throw() family of methods. More in javadocs for Mockito.spy() method.