This may happen if you do something like this:
var anUndefinedVariable;
var rxaSegment = qie.parseHL7String(anUndefinedVariable);
or
var rxaSegment = qie.parseHL7String(someFunctionThatReturnsUndefined());
The 'und' is short for 'undefined'. Passing an undefined variable or calling something that might return an undefined value will cause this.
See also:
What is a MessageModelException: Unknown segment: 'xxx' error?