We had same issues with Java 13. I just overrode Mockito Byte Buddy dependency to a newer version (1.10.3), and it solved inline mock failures in our tests.

Submitted the upgrade in Mockito itself: #1828 Please do not mock Inputstream, but create a real example with fake data instead. That said, we will update Byte Buddy to be compatible with Java 13. Thank you @TimvdLippe ! Would it be possible please to push this release to Maven Central? Creating inline mocks of _any_ classes compiled with javac 13 does not work, not just platform classes like InputStream, so I think this release can be considered 'notable' according to the release policy. It is being released: https://github.com/mockito/mockito/commit/d7c5ac446f156ba6fa913e041441ba4f566f3749 Wonderful, thanks! Thanks everyone for the quick response :-)
Cannot mock this class: class java.io.InputStream with Java 13
## Versions
* Mockito: 3.1.0
* JDK: 13.0.1+9
* OS: macOS 10.13
* XCode: 10.1-19

## Code causing the error
```
InputStream in = mock(InputStream.class);
```

## Error:
```
[ERROR] Tests run: 1, Failures: 0, Errors: 1, Skipped: 0, Time elapsed: 0.015 s <<< FAILURE! - in uk.gov.nationalarchives.droid.core.interfaces.archive.ZipEntryRequestFactoryTest
[ERROR] testNewZipRequest(uk.gov.nationalarchives.droid.core.interfaces.archive.ZipEntryRequestFactoryTest)  Time elapsed: 0.005 s  <<< ERROR!
org.mockito.exceptions.base.MockitoException: 
Mockito cannot mock this class: class java.io.InputStream.
If you're not sure why you're getting this error, please report to the mailing list.
Java               : 13
JVM vendor name    : Oracle Corporation
JVM vendor version : 13.0.1+9
JVM name           : OpenJDK 64-Bit Server VM
JVM version        : 13.0.1+9
JVM info           : mixed mode, sharing
OS name            : Mac OS X
OS version         : 10.13.6
You are seeing this disclaimer because Mockito is configured to create inlined mocks.
You can learn about inline mocks and their limitations under item #39 of the Mockito class javadoc.
Underlying exception : org.mockito.exceptions.base.MockitoException: Could not modify all classes [class java.lang.Object, class java.io.InputStream, interface java.lang.AutoCloseable, interface java.io.Closeable]
	at uk.gov.nationalarchives.droid.core.interfaces.archive.ZipEntryRequestFactoryTest.testNewZipRequest(ZipEntryRequestFactoryTest.java:70)
Caused by: org.mockito.exceptions.base.MockitoException: Could not modify all classes [class java.lang.Object, class java.io.InputStream, interface java.lang.AutoCloseable, interface java.io.Closeable]
	at uk.gov.nationalarchives.droid.core.interfaces.archive.ZipEntryRequestFactoryTest.testNewZipRequest(ZipEntryRequestFactoryTest.java:70)
Caused by: java.lang.IllegalStateException: 
Byte Buddy could not instrument all classes within the mock's type hierarchy
This problem should never occur for javac-compiled classes. This problem has been observed for classes that are:
 - Compiled by older versions of scalac
 - Classes that are part of the Android distribution
	at uk.gov.nationalarchives.droid.core.interfaces.archive.ZipEntryRequestFactoryTest.testNewZipRequest(ZipEntryRequestFactoryTest.java:70)
Caused by: java.lang.IllegalArgumentException: Unsupported class file major version 57
	at uk.gov.nationalarchives.droid.core.interfaces.archive.ZipEntryRequestFactoryTest.testNewZipRequest(ZipEntryRequestFactoryTest.java:70)
```


gradle/dependencies.gradle
gradle/dependencies.gradle
diff --git a/gradle/dependencies.gradle b/gradle/dependencies.gradle
index 79cc490052..0e6b448762 100644
--- a/gradle/dependencies.gradle
+++ b/gradle/dependencies.gradle
@@ -4,7 +4,7 @@ ext {
 
 def versions = [:]
 
-versions.bytebuddy = '1.10.2'
+versions.bytebuddy = '1.10.3'
 versions.junitJupiter = '5.4.2'
 versions.errorprone = '2.3.2'
 
