Section "16.1 Multiple Source Documents" of XSLT describes how items in the $uri-sequence parameter are processed.  It covers the case of:

1. an item in $uri-sequence that is an instance of xs:string, xs:anyURI, or xs:untypedAtomic, 

2. an item in $uri-sequence that is a node

It appears to omit what happens in the case that an item in $uri-sequence is not an instance of xs:string, xs:anyURI, xs:untypedAtomic and is not a node.

e.g. <xsl:value-of select="document(123)" />

While the spec states that in case (2) that the "result must be a sequence whose items are all instances of xs:string, xs:anyURI, or xs:untypedAtomic." it does not state what error should be raised if this is not ther case.