Bug in Daffodil? Unparse Error: Data too long by 8 bits. Unable to truncate.

[Costello, Roger L.]

Hello DFDL community,

My input consists of a string containing one to five characters. Each character 
must be a digit character. Here is a sample input:

        99999

My DFDL schema uses the xs:unsignedInt datatype to constrain the string to 
digits. It uses dfdl:length=5 to constrain the length of the string. 

I am trying to be very precise in my wording. Is the above wording correct?

Here is my DFDL schema:

<xs:element name="input">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="value" 
                type="xs:unsignedInt" 
                dfdl:lengthKind="explicit" 
                dfdl:length="5" 
                dfdl:lengthUnits="characters" />
        </xs:sequence>
    </xs:complexType>
</xs:element>

Parsing the input 99999 produces this XML:

<input>
  <value>99999</value>
</input>

Unparsing produces this error message:

[error] Unparse Error: Data too long by 8 bits. Unable to truncate.

Here is the output of unparsing:

99,999

Why does unparsing produce that error message? Is it a bug in Daffodil?

/Roger