Running extcheck with arbitrary rt.jar, for example rt.jar yields an IO exception

Observed on Solaris and Windows (did not check on Linux)

> extcheck -J-showversion -verbose /local/jdk6_14/jre/lib/rt.jar
java version "1.6.0_14"
Java(TM) SE Runtime Environment (build 1.6.0_14-b08)
Java HotSpot(TM) Server VM (build 14.0-b16, mixed mode)
 
Target file:/local/jdk6_14/jre/lib/rt.jar
Specification title:Java Platform API Specification
Specification version:1.6
Specification vendor:Sun Microsystems, Inc.
Implementation version:1.6.0_14
Implementation vendor:Sun Microsystems, Inc.
 
Comparing with file:/local/jdk6_14/jre/lib/ext/sunjce_provider.jar
Comparing with file:/local/jdk6_14/jre/lib/ext/sunpkcs11.jar
Comparing with file:/local/jdk6_14/jre/lib/ext/dnsns.jar
Comparing with file:/local/jdk6_14/jre/lib/ext/localedata.jar
Comparing with file:/local/jdk6_14/jre/lib/ext/meta-index
IO Exception occurred

This is observed with JDK 6 and latest jdk7