DESCRIPTION is not valid for INPUT or OUTPUT.

I am getting Error when trying to parse a yang file.

my yang is:
module zws {
namespace "a:b:c";
prefix zws;
revision 2010-09-24

{ description "Initial revision."; }

rpc method1 {
input {
description "input des.";
leaf param

{ type string; }
}
output {
description "output des.";
leaf param { type string; }

}
}
}

the error is:
10:55:18.052 [main] WARN o.o.y.y.p.i.YangStatementParserListenerImpl - DESCRIPTION is not valid for INPUT. Error in module zws (QNameModule

{ns=a:b:c, rev=2010-09-24}

)
org.opendaylight.yangtools.yang.parser.spi.meta.InvalidSubstatementException: DESCRIPTION is not valid for INPUT. Error in module zws (QNameModule

{ns=a:b:c, rev=2010-09-24}

)
at org.opendaylight.yangtools.yang.parser.spi.SubstatementValidator.validate(SubstatementValidator.java:92) ~[classes/:na]
...
10:55:18.053 [main] WARN o.o.y.y.p.i.YangStatementParserListenerImpl - DESCRIPTION is not valid for OUTPUT. Error in module zws (QNameModule

{ns=a:b:c, rev=2010-09-24}

)
org.opendaylight.yangtools.yang.parser.spi.meta.InvalidSubstatementException: DESCRIPTION is not valid for OUTPUT. Error in module zws (QNameModule

{ns=a:b:c, rev=2010-09-24}

)
at org.opendaylight.yangtools.yang.parser.spi.SubstatementValidator.validate(SubstatementValidator.java:92) ~[classes/:na]
...

I think there may be have descriptions in the input or ouput node of rpc method. This may be a bug, there may be something wrong in the code of yangtools project.