Real method getting called when Mockito doNothing method is called

I am trying to mock the class KeyStore. After mocking I do not want anything to happen when the load method if it has been called. Therefore I wrote the below lines to achieve this.

        @PrepareForTest(KeyStoreFactory.class)
        @Test
        public void should_verify_signature_when_verifySignature_called_with_fileName_and_certificate_details_in_verifySignature_method() throws Exception {
            PowerMockito.mockStatic(KeyStoreFactory.class);

            KeyStore keyStoreMock = PowerMockito.mock(KeyStore.class);
            PowerMockito.when(KeyStoreFactory.getInstance(anyString(), anyString())).thenReturn(keyStoreMock);
            Mockito.doNothing().when(keyStoreMock).load(Mockito.any(InputStream.class), Mockito.any(char[].class));
            Certificate certificateMock = Mockito.mock(Certificate.class);
            when(keyStoreMock.getCertificate(anyString())).thenReturn(certificateMock);
            boolean result = signatureUtil.verifySignature("src//test//java//Updates.zip.signed.pkcs7"
                    , "src//test//java//Updates-retrieved.zip", "Windows-MY,SunMSCAPI,someName");
            Assert.assertTrue(result);

        }

But the load method was throwing null pointer exception. Then when I debug I found out that the real method is getting called though I have specified for mockito to not to. What am I doing wrong here? Please advice.