calling extension java functions. ( Saxon Vs Xalan)

I have a java extension method that i call from my XSL
the way i call is
<xsl:value-of select="str-utils:upperCase($mystring)"/>
it works fine with Xalan but fails with Saxon.
saxon gives
 Transform failed:  At xsl:value-of on line 264 of
file:/tests/saxon-test.xsl: No method matching upperCase with 1 parameter
found in class com.myutils.util.StringUtils

the namespace declaration is:
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
                xmlns:saxon="http://icl.com/saxon" xmlns:str-utils =
"com.myutils.utils.StringUtils"
                extension-element-prefixes="saxon str-utils"  version="1.0">
It however doesnt give any error with the Xalan ( with a different namespace
declaration)
Am i calling it correctly?? is there any difference in the way you call
extension functions with xalan and saxon?
Thanks

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