problemas al concatenar un selectedIndex en un onChange

[Alexis_Mejias]

Código:

<select name="id_ddr" id="id_ddr" class="tit3" 
onChange="javascript:document.location='rpt_precipitaciones.php?p=2&id_ddr=' + this.value;">
<option value="0" selected="selected">-SELECCIONAR-</option>
<option value="154">LAREDO</option>
<option value="155">DIAZ ORDAZ</option>
<option value="156">CONTROL</option>
<option value="157">SAN FERNANDO</option>
<option value="158">ABASOLO</option>
<option value="159">VICTORIA</option>
<option value="160">JAUMAVE</option>
<option value="161">MANTE</option>
<option value="162">GONZALEZ</option>
</select>

Y ahí que tal?