Adding List to Map

I am trying to add a list to a map ,but iam getting the following exception

Caused by: java.lang.RuntimeException: Cannot create a map entry without a key specified.

Here is the pieced of xpath iam using in the service

Location : /process_data/mapofConvertedPdfs[@id='listOfConvertedPdfs']
Expression : /process_data/convertedListofPdfs

mapofConvertedPdfs is the map which has subtype as document
convertedListofPdfs is the list which has subtype as document

can anyone please help me to get this resolved