B1iF V2 Select Not Working

[Moriaty]

<Response>
  <returnData>
    <xsl:value-of select="/bfa:call/bfa:outbound/bfa:payload/uplt:httaError/uplt:returnData"> 
    </xsl:value-of>
  </returnData>
</Response>

is returning empty:

<Response xmlns:uplt="urn.com.sap.b1i.xcellerator:upltdoc">
  <returnData/>
</Response>

but the outbound message contains the following error that I would expect to see returned above:

<returnData>{ "error" : { "code" : -2028, "message" : { "lang" : "en-us", "value" : "Invalid BP code [OINV.CardCode] , 'X40000'" } } } </returnData>

I am very new to integration, so sorry if I have left out pertinent details. Any help would be appreciated.

[Moriaty · Accepted solution]

Adding xmlns:xs="http://www.w3.org/2001/XMLSchema" fixed it.