diff --git a/Data/RTree/Base.hs b/Data/RTree/Base.hs
--- a/Data/RTree/Base.hs
+++ b/Data/RTree/Base.hs
@@ -82,6 +82,7 @@
     | Empty
     deriving (Show, Eq, Functor, Typeable, Generic)
 
+-- | It is possible, to change these constants, but the tree won't be space optimal anymore.
 m, n :: Int
 m = 2
 n = 4
@@ -361,7 +362,8 @@
 
 -- | Unifies the first and the second tree into one. The combining function is used for elemets which exists in both trees.
 unionWith :: (a -> a -> a) -> RTree a -> RTree a -> RTree a
-unionWith _ Empty Empty = Empty
+unionWith _ l     Empty    = l
+unionWith _ Empty r        = r
 unionWith f t1 t2
     | depth t1 <= depth t2 = foldr (uncurry (insertWith f)) t2 (toList t1)
     | otherwise            = unionWith f t2 t1
@@ -384,6 +386,7 @@
 -- ---------------
 
 isValid :: Show b => b -> RTree a -> Bool
+isValid _ Empty = True
 isValid _ Leaf{} = True
 isValid context x = case L.length c >= m && L.length c <= n && (and $ (isValid context) <$> c) && (isBalanced x) of
     True -> True
@@ -422,7 +425,8 @@
 -- ----------------------
 
 depth :: RTree a -> Int
-depth (Leaf _ _ ) = 0
+depth Empty = 0
+depth (Leaf _ _ ) = 1
 depth t = 1 + (depth $ head $ getChildren t)
 
 -- | returns the number of elements in a tree
diff --git a/Data/RTree/MBB.hs b/Data/RTree/MBB.hs
--- a/Data/RTree/MBB.hs
+++ b/Data/RTree/MBB.hs
@@ -28,7 +28,6 @@
 where
 
 import Data.Binary
-import Data.Monoid
 
 import GHC.Generics (Generic)  
 
@@ -78,9 +77,3 @@
   show (MBB ulx uly brx bry) = concat ["mbb ", show ulx, " ", show uly, " ", show brx, " ", show bry]
 
 instance Binary MBB
-
--- | mconcat will fail for empty lists. 
-instance Monoid MBB where
-  mempty = MBB 0.0 0.0 0.0 0.0
-  mappend = unionMBB
-  mconcat = unionsMBB
diff --git a/data-r-tree.cabal b/data-r-tree.cabal
--- a/data-r-tree.cabal
+++ b/data-r-tree.cabal
@@ -1,5 +1,5 @@
 name:                data-r-tree
-version:             0.0.2.0
+version:             0.0.3.0
 synopsis:            R-Tree is a spatial data structure similar to Quadtrees or B-Trees.
 description:         R-Tree is a spatial data structure similar to Quadtrees or B-Trees.
   
@@ -11,7 +11,7 @@
 author:              Sebastian Philipp, Birte Wagner
 maintainer:          sebastian@spawnhost.de
 copyright:           Sebastian Philipp, Birte Wagner
-category:            Data
+category:            Data Structures
 build-type:          Simple
 
 -- extra-source-files:  
diff --git a/test/RTreeProperties.hs b/test/RTreeProperties.hs
--- a/test/RTreeProperties.hs
+++ b/test/RTreeProperties.hs
@@ -143,7 +143,9 @@
 test_union :: Assertion
 test_union = do
     union empty empty `eqRt` (empty :: RTree ())
-    union tu_2 tu_1 `eqRt` tu_2
+    union tu_2 tu_1   `eqRt` tu_2
+    union t_1 empty   `eqRt` t_1
+    union empty t_1   `eqRt` t_1
 
 test_unionWith :: Assertion
 test_unionWith = do