diff --git a/HsWebots.cabal b/HsWebots.cabal
--- a/HsWebots.cabal
+++ b/HsWebots.cabal
@@ -4,10 +4,10 @@
 --
 -- see: https://github.com/sol/hpack
 --
--- hash: 3b6e21d4ea12644e25ba1c2ef63bf2354c2a9c2624c04cf194be97b696647a81
+-- hash: d7399ff5367c55e2b2afaaae32dcf8116890e0a71844b15a4b105fd4f5564ee8
 
 name:           HsWebots
-version:        0.1.0.1
+version:        0.1.1.0
 synopsis:       Webots bindings for Haskell
 description:    Please see the README on GitHub at <https://github.com/junjihashimoto/HsWebots#readme>
 category:       Robotics
diff --git a/src/Webots/Supervisor.hs b/src/Webots/Supervisor.hs
--- a/src/Webots/Supervisor.hs
+++ b/src/Webots/Supervisor.hs
@@ -141,10 +141,14 @@
 wb_supervisor_node_get_type node =
    [C.exp| WbNodeType { wb_supervisor_node_get_type($(WbNodeRef node)) } |]
 
-wb_supervisor_node_get_field :: WbNodeRef -> String -> IO WbFieldRef 
-wb_supervisor_node_get_field node field_name =
-  withCString field_name $ \field_name' -> 
-   [C.exp| WbFieldRef { wb_supervisor_node_get_field($(WbNodeRef node), $(const char* field_name')) } |]
+wb_supervisor_node_get_field :: WbNodeRef -> String -> IO (Maybe WbFieldRef)
+wb_supervisor_node_get_field node field_name = do
+  ptr <- withCString field_name $ \field_name' -> 
+           [C.exp| WbFieldRef { wb_supervisor_node_get_field($(WbNodeRef node), $(const char* field_name')) } |]
+  if ptr == nullPtr
+  then return Nothing
+  else return $ Just ptr
+    
 
 wb_supervisor_node_remove :: WbNodeRef -> IO () 
 wb_supervisor_node_remove node =
@@ -500,10 +504,8 @@
 fieldGetMfNode :: WbFieldRef -> CInt -> IO WbNodeRef
 fieldGetMfNode ptr i = [C.exp| WbNodeRef { wb_supervisor_field_get_mf_node($(WbFieldRef ptr), $(int i)) } |]
 
-getField :: WbNodeRef -> String -> IO WbFieldRef
-getField ptr str =
-  withCString str $ \cstr -> 
-    [C.exp| WbFieldRef { wb_supervisor_node_get_field($(WbNodeRef ptr), $(char* cstr)) } |]
+getField :: WbNodeRef -> String -> IO (Maybe WbFieldRef)
+getField ptr str = wb_supervisor_node_get_field ptr str
 
 getId :: WbNodeRef -> IO CInt
 getId ptr =
@@ -535,14 +537,20 @@
 getRootNodes :: IO [(WbNodeRef,CInt,String)]
 getRootNodes = do
   root <- getRootNode
-  children <- getField root "children"
-  cnt <- fieldGetCount children
-  forM [0..(cnt-1)] $ \i -> do
-    node <- fieldGetMfNode children i
-    nodeId <- getId node
-    typeName <- getTypeName node
-    return (node,nodeId,typeName)
-
-getNodeFieldByString :: WbNodeRef -> String -> IO String
-getNodeFieldByString node label = getField node label >>= fieldGetSfString
+  mchildren <- getField root "children"
+  case mchildren of
+    Nothing -> return []
+    Just children -> do
+      cnt <- fieldGetCount children
+      forM [0..(cnt-1)] $ \i -> do
+        node <- fieldGetMfNode children i
+        nodeId <- getId node
+        typeName <- getTypeName node
+        return (node,nodeId,typeName)
 
+getNodeFieldByString :: WbNodeRef -> String -> IO (Maybe String)
+getNodeFieldByString node label = do
+  m <- getField node label
+  case m of
+    Nothing -> return Nothing
+    Just field -> fieldGetSfString field >>= return.Just