Search at www.google.fr:
An empty sequence is not allowed as the @select attribute of xsl:analyze-string
That's known issue. See Bug 7976.
In xslt 2.0 you should either check the value before using xsl:analyze-string, or wrap it into string() call.
The problem is addressed in xslt 3.0
a@href@title, b, blockquote@cite, em, i, strike, strong, sub, super, u