Error adding a new WMI counter

When I try to add a new WMI counter using the following entries:

<DEFAULTGRAPH Name="SmtpMessageQueueTotal">
<YAXISTEXT>No of Messages</YAXISTEXT>
<DISPLAYNAME>SMTP Message Queue Total</DISPLAYNAME>
<name>SmtpMessageQueueTotal</name>
<Vendor>WMI</Vendor>
<Description>Total number of messages in the SMTP Message Queue folder</Description>
<oid>SmtpMessageQueueTotal</oid>
<Provider>WMI</Provider>
<type>multiple</type>
<interval>60</interval>
<THRESHOLDENABLED>false</THRESHOLDENABLED>
</DEFAULTGRAPH>

<CATEGORY ID="SmtpMessageQueueTotal" command="SmtpMessageQueueCount.vbs" parseAll="false" ignoreLines="3" description="Returns the total number of messages in the SMTP Queue folder.">
<ENTITY ID="SmtpMessageQueueTotal" storeValue="Total" storeHistory="true" units="%">
<PARSE line="3" token="2" name="Total" />
</ENTITY>
</CATEGORY>

and then try to access the add page I'm faced with a page that simply says "Error!". Not very useful at all I might add. Any thoughts on why the above might cause that?

Thanks,
Colin