signed jar with only META-INF/* inside is not verifiable

If a jar file has only META-INF/MANIFEST.SF, then after signing it, the output jarfile is not verifiable. That is to say, running "jarsigner -verify the.jar" shows "jar is unsigned. (signatures missing or not parsable)".

How to reproduce it:

echo "Key: Value" > manifest
jar cvfm the.jar manifest
jarsigner the.jar me
jarsigner -verify the.jar

* Use the "jar m" trick to create a jar file with no "normal" entry.