Haskell find indices of String in String
===

I'm currently trying to find the indices of a particular string in another string.
So e.g. the result for string ab in "ababa baab ab bla ab" should be 11 and 18. 
If currently have the problem that with my function I also get the index 8  
My function:  

    findSubstringIndices :: String -> String -> [Int]
    findSubstringIndices text pattern = map (add 1) (findIndices (pattern `isPrefixOf`) (tails text))

 