diff --git a/gluturtle.cabal b/gluturtle.cabal
--- a/gluturtle.cabal
+++ b/gluturtle.cabal
@@ -2,7 +2,7 @@
 cabal-version:	>= 1.6
 
 name:		gluturtle
-version:	0.0.37
+version:	0.0.38
 stability:	alpha
 author:		Yoshikuni Jujo <PAF01143@nifty.ne.jp>
 maintainer:	Yoshikuni Jujo <PAF01143@nifty.ne.jp>
diff --git a/src/Graphics/UI/GLUT/Turtle/Move.hs b/src/Graphics/UI/GLUT/Turtle/Move.hs
--- a/src/Graphics/UI/GLUT/Turtle/Move.hs
+++ b/src/Graphics/UI/GLUT/Turtle/Move.hs
@@ -119,6 +119,8 @@
 
 drawTurtle :: Field -> TurtleState -> Double -> Position ->
 	Maybe Position -> IO ()
-drawTurtle f ts@TurtleState{fillcolor = fclr, pencolor = clr} dir pos = maybe
-	(drawCharacter f fclr clr (makeShape ts dir pos) (pensize ts))
-	(drawCharacterAndLine f fclr clr (makeShape ts dir pos) (pensize ts) pos)
+drawTurtle f ts@TurtleState{fillcolor = fclr, pencolor = clr} dir pos
+	| [] <- makeShape ts dir pos = error $ show ts
+	| otherwise = maybe
+		(drawCharacter f fclr clr (makeShape ts dir pos) (pensize ts))
+		(drawCharacterAndLine f fclr clr (makeShape ts dir pos) (pensize ts) pos)
diff --git a/src/Graphics/UI/GLUT/Turtle/State.hs b/src/Graphics/UI/GLUT/Turtle/State.hs
--- a/src/Graphics/UI/GLUT/Turtle/State.hs
+++ b/src/Graphics/UI/GLUT/Turtle/State.hs
@@ -33,6 +33,7 @@
 	positionStep :: Maybe Double,
 	directionStep :: Maybe Double,
 	interval :: Int}
+	deriving Show
 
 initTurtleState :: TurtleState
 initTurtleState = TurtleState {