suppressing certain values in a select element

Hello, 

I would like to suppress certain values, which are dynamically created through a itemset, depending on the element content. I just want to display all element values from the element tm:baseNameString where the content is not empty. 
I tried it with 

<p class="padding">
	<xforms:select1 ref="tm:instanceOf/tm:topicRef/@xlink:href">
		<xforms:label>Typ of the Association: </xforms:label>
		<xforms:hint>Please choose a name for your association</xforms:hint>
		<xforms:itemset nodeset="//tm:topicMap/tm:topic/tm:baseName/tm:baseNameString[string-length()!= 0]">
			<xforms:label ref="." />
			<xforms:value ref="../../@id" />
		</xforms:itemset>
	</xforms:select1>
</p>

but it doesn't work out, because it seems to be a invalid binding expression. 
I would like to know if there is another way to achieve this or if it is supposed to work.
Thanks for the help.

wbr,
Roman

Roman Huditsch (hRHU )
Developer .:. Information & Application Engineering 
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