When a supplied value for a list variable is not coercible into the required type specified by the operation the error message can be confusing at times:

While the variable is correctly trapped and the query failed, the message indicates that a value of null was not valid. This is because the list resolver, when it fails to coerce a value, returns null. The list value resolver should throw an UnresolvedValueException like the scalar resolvers do and supply the appropriate information indicating the reason for failure.