Here is my problem with using DITA-OT.

I am just starting to use the "Filtering" feature when I encounter the following issue:

Here is how I run the DITA-OT:

java -jar lib/dost.jar /i:/conditional-text.ditamap /filter:plumbers.ditaval /outdir:. /transtype:pdf2

And here is the result: 
BUILD FAILED
D:\DITA-OT\build.xml:44: The following error occurred while executing this line:
D:\DITA-OT\plugins\org.dita.base\build_preprocess.xml:24: Failed to run pipeline: [DOTJ012F][FATAL] Failed to parse 

the input file 'D:\DITA-OT\conditional-text.ditamap'. The XML parser reported the following error: : conditional-

text.ditamap Line 2:cvc-elt.1.a: Cannot find the declaration of element 'val'.

Here is the content of my conditional-text.ditamap

<?xml version="1.0" encoding="UTF-8"?>
<val>
<prop att="audience" val="electricians" action="include"/>
<prop att="audience" val="plumbers" action="exclude"/>
</val>

Can anyone tell me how to fix this?Thanks!