Prefix not resolving in RDF when ID/NAME has period (eg: #aaaaaaa.bbbbbb)

[NovasTaylor]

I am creating RDF in OpenRefine using the DERI extension. I use a prefix "foo" to reference an external list of coded terms:
@Prefix foo: http://www.codesource.foo/sdtm-terminology# .
I refer to this list using rdfs:seeAlso in order to tie my local codelist to the external source. An example value for my rdfs:seeAlso would be:
http://www.codesource.foo/sdtm-terminology#C66731.C16576

I want my resulting RDF use of the prefix "foo", as in:
my-code:sex-F a skos:Concept , sdmx:Concept , my-code:Sex ;
rdfs:seeAlso foo:C66731.C16576 .
I used OpenRefine to build the value as a "Constant Value" , "as a URI". My Constant Value is specified as:
http://www.codesource.foo/sdtm-terminology#C66731.C16576

The period in the value C66731.C16576 causes my RDF to not use the prefix, instead giving me:

crmd-code:sex-F a skos:Concept , sdmx:Concept , crmd-code:Sex ;
rdfs:seeAlso http://rdf.cdisc.org/sdtm-terminology#C66731.C16576 .

Is there a work-around so I can use the prefix? Is there a way to "escape" the period? If I remove the period the prefix resolves perfectly as:
rdfs:seeAlso sdtmct:C66731C16576 .
I have no control over the the "dot" terminology in my external reference. Why is "dot" not acceptable? The HTML spec shows periods are acceptable for ID and NAME tokens, unless I am missing something.
http://www.w3.org/TR/html4/types.html#type-id
Thank you for any advice you may be able to provide.
Tim W.