Description of fn:index-of contains this paragraph:

The items in the sequence $seqParam are compared with $srchParam under the 
rules for the eq operator. Values that cannot be compared, i.e. the eq operator 
is not defined for their types, are considered to be distinct. If an item 
compares equal, then the position of that item in the sequence $srchParam is 
included in the result.

In the final sentence, $srchParam should be $seqParam.