Problems validating a schemaconstrained variable

Hi - I’m trying to use schema validatation to ensure a string variable always contains an integer. However, pub.schema:validate is consistently throwing an exception at me.

Here’s how I set the constraint: In WM Developer’s Input/Output tab, I right-click the variable (its an input param to a service), select Properties/Constraints, then ‘Browse’ and select the ‘integer’ element in the ‘pub.schema.w3c.sep_2000:datatypes’ schema set (WmPublic package). Within the service, I then call pub.schema:validate with the variable mapped to the ‘object’ input and conformsTo=‘pub.schema.w3c.sep_2000:datatype’

However, even when the input string variable contains an integer (say: “2”), I consistenly get this error:
“[B2BSERV.0062.9024] Integration Server does not support this type of object in validation 2”

Is there something I’m doing wrong?