Why can't the element referenced by dfdl:choiceBranchKey use dfdl:inputValueCalc?

[Costello, Roger L.]

Hello DFDL community,

If my input file is this:

MZ

Then I want the output XML to be this:

<input>
    <Magic-Number>MZ</Magic-Number>
   <Executable>Windows executable</Executable>
</input>

If my input file is this:

PNG

Then I want the output XML to be this:

<input>
    <Magic-Number>PNG</Magic-Number>
   <Image>PNG image</Image>
</input>

If my input file is this:

PK

Then I want the output XML to be this:

<input>
    <Magic-Number>PK</Magic-Number>
   <Archive>Zip archive</Archive>
</input>

If the input is none of those, then I want the output XML to be this:

<input>
    <Magic-Number>...</Magic-Number>
    <Unrecognized>Unrecognized magic number</Unrecognized>
</input>

Below is my DFDL schema. It produces this error:

element reference {}Executable cannot have the dfdl:inputValueCalc property.

Why am I getting that error?  /Roger

<xs:element name="input">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="Magic-Number" type="xs:string" />
            <xs:choice>
                <xs:choice dfdl:choiceDispatchKey="{./Magic-Number}">
                    <xs:element ref="Executable" dfdl:choiceBranchKey="MZ" />
                    <xs:element ref="Image" dfdl:choiceBranchKey="PNG" />
                    <xs:element ref="Archive" dfdl:choiceBranchKey="PK" />
                </xs:choice>
            </xs:choice>
            <xs:element name="Unrecognized" type="xs:string" 
dfdl:inputValueCalc="{'Unrecognized magic number'}" />
        </xs:sequence>
    </xs:complexType>
</xs:element>

<xs:element name="Executable" type="xs:string" dfdl:inputValueCalc="{'Windows 
executable'}" />
<xs:element name="Image" type="xs:string" dfdl:inputValueCalc="{'PNG image'}" />
<xs:element name="Archive" type="xs:string" dfdl:inputValueCalc="{'Zip 
archive'}" />