using variables in functions

I have no trouble passing variables (initialized as strings using
select="expression")
as $variable into some XSLT functions (e.g. translate() ), but functions seem
to throw
a typing  error (e.g. escape-uri(), and replace() ).

I am using Java5 parsers etc:
a typical xslt call is
<xsl:variable name="a" select="string(./rdfs:label)" />
<xsl:variable name="b" select="replace($a, '%', '%25') /> ( {$a} doesnt work
either )

a typical error reported by the java program is:
Could not compile stylesheet
Error checking type of the expression 'funcall(replace
[variable-ref(a/string),
literal-expr(%), literalexpr(%25)]) ' . '

this works for translate($a, '%', '%25') : what's the difference?

if I could get escape-uri to work it must be the cool function to avoid all
the horrible
nested substring/replacements to url-encode.

thanks anybody
Trevor Paterson
Scotland