fix a small issue in NaturalStaticOrder

5 jobs for master in 26 minutes and 9 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Build
passed #854
jars-j8

00:00:32

passed #853
java8

00:12:53

 
  Test
passed #857
java7

00:12:30

failed #856
allowed to fail
java9

00:00:12

failed #855
java7

00:03:50

 
Name Stage Failure
failed
java9 Test There is an unknown failure, please try again
	at java.base/sun.security.validator.PKIXValidator.<init>(PKIXValidator.java:86)
... 72 more
2447 [ERROR]
2447 [ERROR] Re-run Maven using the -X switch to enable full debug logging.
2447 [ERROR]
2447 [ERROR] For more information about the errors and possible solutions, please read the following articles:
2448 [ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/ProjectBuildingException
2448 [ERROR] [Help 2] http://cwiki.apache.org/confluence/display/MAVEN/UnresolvableModelException
ERROR: Job failed: exit code 1