unclear error on converting node type from XPath

The below error is what happens when you attempt to use an XPath result in SelectIn if the XPath didn't actually select any nodes. The reason is because the asNode function of TypeEval will check if any nodes were selected, resulting in ?null? if none. This can be very tricky to get an appropriate error message -- it is related to any type conversion, not just for selectIn.

00000006-00 CHECKPOINT-FAIL ScriptParsing-TypeConvert:
        Parameter: Node
        TargetType: Node
        Source: %DescNode%
        SourceObject: testplan.auto.htmlunit.ResponseState$XPath@28f337b <testplan.auto.htmlunit.ResponseState$XPath>
        Error: Parameter 'Node' cannot be converted to 'Node'
        TestUnit: wicksaircraft.CollectCategory
AutoStack: in {Unknown} in htmlunit at step 0
        /home/projects/Current/eversystems/elance/wayman/wicksaircraft/CollectCategory.test:13: Set.selectIn %DescNode% .//tr[1]
        /home/projects/Current/eversystems/elance/wayman/wicksaircraft/GetCatLinks.test:6: call CollectCategory
        /home/projects/Current/eversystems/elance/wayman/wicksaircraft/GetCategories.test:23: call GetCatLinks
        /home/projects/Current/eversystems/elance/wayman/funcs/CollectSite.test:32: call %Cmds.Site%.GetCategories