FOTY0001: Type Error

I have encountered the below error during runtime many a time:

ORABPEL-09500
XPath expression failed to execute.
Error while processing xpath expression, the expression is "ora:processXSLT('Transformation_5.xsl',bpws:getVariableData('inputVariable','payload'))", the reason is FOTY0001: type error. Please verify the xpath query.

This error, though mentions the reason for failure was due to a xpath expression, when one executes the xslt using xalan or test option of Jdeveloper, the xslt works fine. I later found that the part name was missing in the transformation node where the xsl was used. This could be because, the bpel file was edited prematurely even before the details of the node were loaded or could be an actual issue with the schema (in case of first timers)!

Always check if the part name in the transformation node are present and are correct before debugging further on this error. I found the this link useful when I was debugging.

Please note this is only one such FOTY0001 error (most common) and one such solution which worked for me. There could be many other reasons for the error in different scenarios and hence different resolutions.

Cheers,
-AR