Scala: mutable.Map[Any, mutable.Map[Any, IndexedSeq[Any]] issue - scala

Question

I have a tiny problem when trying to put an item in my second mutable map.
My objective: gather some elements located in many different xml files, and organise them in the hierarchy they belong (the files are an unstructured mess, with categories being given in seemingly no logical order).
Those elements are: the level in hierarchy of the category (1 - x, 1 is top level) as iLevel, the category code as catCode, its name, and if need be, the name of its parents (all names located in namesCategories).
val categoryMap = mutable.Map.empty[Int, mutable.Map[String, IndexedSeq[String]]]
...
//Before: search in a first file links to other files
// for each category file found, will treat it and store it for further treatement.
matches.foreach{f =>
....
//Will search for a specific regex, and for each matches store what we are interested in
matchesCat.foreach{t =>
sCat = t.replaceFirst((system_env + """\S{4}"""), "")
//iLevel given by the number of '/' remaining in the string
iLevel = sCat.count(_ == '/')
//reset catCode and namesCategories
catCode = ""
namesCategories.clear()
//Search and extract the datas from sCat using premade regex patterns
sCat match {
case patternCatCode(codeCat) => catCode = s"$codeCat"
}
//remove the category code to prepare to extract names
sCat.replace(patternCatCode.toString(), "")
//extract names
do {
sCat match {
case patternCatNames(name) => namesCategories += s"$name"
}
sCat.replace(patternCatNames.toString(), "")
}while(sCat!="")
// create the level entry if it doesn't exist
if(!(categoryMap.contains(iLevel))) {
categoryMap.put(iLevel, mutable.Map.empty[String, IndexedSeq[String]])
}
//Try to add my cat code and the names, which must be in order for further treatment, to my map
categoryMap(iLevel).put(catCode, namesCategories.clone())
}
}
}
Problem:
Type mismatch, expected: IndexedSeq[String], actual: mutable.Builder[String, IndexedSeq[String]]
As Travis Brown kindly noted, I have an issue with a type mismatch, but I don't know how to fix that and get the general idea working.
I tried to keep the code to only what is relevant here, if more is needed I'll edit again.
Any tips?
Thanks for your help