Return type problem.

class ComplexCurveSecuritiesCollection[T <: SecurityImpl,X<:TermStructure](private val subcollections: IndexedSeq[GenericCurveSecuritiesCollection[T,X]]) extends CurveSecuritiesCollection[T] {
  type SubType = ComplexCurveSecuritiesCollection[T,X]
  type TermStructureType = X
  def filter(f: (T) => Boolean) = {    val newCollections = subcollections.map(item=>item.filter(f));     new ComplexCurveSecuritiesCollection[T,X](newCollections);  }
}
error: type mismatch;found   : IndexedSeq[com.gottex.gottware.common.datamodels.finance.collections.curves.CurveSecuritiesCollection.GenericCurveSecuritiesCollection[T,X]#SubType] required: IndexedSeq[com.gottex.gottware.common.datamodels.finance.collections.curves.CurveSecuritiesCollection.GenericCurveSecuritiesCollection[T,X]]new ComplexCurveSecuritiesCollection[T,X](newCollections);
where 
trait CurveSecuritiesCollection[T <: SecurityImpl] extends RepriceableAndPublisheableCollection with java.lang.Iterable[T] {
  type SubType <: CurveSecuritiesCollection[T]


  def filter(f: (T) => Boolean): SubType
}
What is wrong in my code?
Best RegardsEdmondo Porcu