diff --git a/alib2data/src/tree/TreeFeatures.h b/alib2data/src/tree/TreeFeatures.h
index 728068e4de8a94ebff3acb6af0db863ac8e51151..2828aed74b819760157b61b4277ad9705c710d12 100644
--- a/alib2data/src/tree/TreeFeatures.h
+++ b/alib2data/src/tree/TreeFeatures.h
@@ -30,6 +30,8 @@ enum class FEATURES {
 };
 
 class Tree;
+class RankedTreeWrapper;
+class UnrankedTreeWrapper;
 class TreeBase;
 
 template < class SymbolType = DefaultSymbolType, class RankType = DefaultRankType >