Extension functions were designed to return an error code -- there is even
a parameter in there definition which specifies which error codes they
return. But due to a cut and paste typo, the extension function type
returns a void:

typedef void (JNICALL *jvmtiExtensionFunction)
                (const jvmtiEnv* env, ...);

this should be:


typedef jvmtiError (JNICALL *jvmtiExtensionFunction)
                (const jvmtiEnv* env, ...);