Saxon XSLT and XQuery Processor

I am using a StringBuffer to accumulate Strings in a
stylesheet.

When I run
<xsl:value-of select="stringBuffer:append($sentList,
concat('x',
''y'))"/>

I get the error :
There are several Java methods that match equally well.

StringBuffer.append takes a number of argument types.
Among these are
String and Object. I believe the problem lies with the
conversionPreferences method in StringValue. I made
the change
indicated below and my stylesheet ran as expected.

public int conversionPreference(Class required) {

// if (required.isAssignableFrom(StringValue.class))
return 0;

...

if (required==String.class) return 0;
if (required==Object.class) return 1;

// put this after the Object test. if required ==
Object
// then this test always returns true even when
there
// is a more specific value match.

if (required.isAssignableFrom(StringValue.class))
return 0;
return Integer.MAX_VALUE;
}