oraext:index-within-string throws internal xpath extension error

Using oraext:index-within-string is not pretty simple as it looks so . If you are using

oraext:index-within-string(($inputVariable.payload/client:DocId),'.')

Unlike builtin xpath functions which have internal type conversion this throws an error. Try using 

oraext:index-within-string(string($inputVariable.payload/client:DocId),'.')

Instead which works . Please note the conversion of string here .

This is applicable to oracle bpel assign activity .