diff --git a/src/server/MainFastCGI.hs b/src/server/MainFastCGI.hs
index 7c02924d6..cd644c9ad 100644
--- a/src/server/MainFastCGI.hs
+++ b/src/server/MainFastCGI.hs
@@ -1,3 +1,5 @@
+{-# LANGUAGE DeriveDataTypeable #-}
+
 import PGF
 import FastCGIUtils
 
@@ -5,6 +7,9 @@ import Network.CGI hiding (Language)
 import Text.JSON
 import qualified Codec.Binary.UTF8.String as UTF8 (encodeString)
 
+import Control.Exception
+import Control.Monad
+import Data.Dynamic
 import Data.Maybe
 
 
@@ -23,7 +28,7 @@ instance JSON a => JSON (Record a) where
 main :: IO ()
 main = do initFastCGI
           r <- newDataRef
-          loopFastCGI (fcgiMain r)
+          loopFastCGI (handleErrors (handleCGIErrors (fcgiMain r)))
 
 fcgiMain :: DataRef PGF -> CGI CGIResult
 fcgiMain ref = getData readPGF ref grammarFile >>= cgiMain
@@ -32,22 +37,52 @@ cgiMain :: PGF -> CGI CGIResult
 cgiMain pgf =
     do path <- pathInfo
        case path of
-         "/translate" -> do input <- fmap (fromMaybe "") $ getInput "input"
-                            mcat <- getInput "cat"
+         "/translate" -> do input <- liftM (fromMaybe "") $ getInput "input"
+                            mcat  <- getInput "cat"
                             mfrom <- getInput "from"
-                            mto <- getInput "to"
+                            mto   <- getInput "to"
+                            maybe (return ()) (checkCategory pgf) mcat
+                            maybe (return ()) (checkLanguage pgf) mfrom
+                            maybe (return ()) (checkLanguage pgf) mto
                             outputJSON $ translate pgf input mcat mfrom mto
          _ -> outputNotFound path
 
 outputJSON :: JSON a => a -> CGI CGIResult
 outputJSON x = do setHeader "Content-Type" "text/json; charset=utf-8"
-                  output $ UTF8.encodeString $ encode x
+                  outputStrict $ UTF8.encodeString $ encode x
+
+outputStrict :: String -> CGI CGIResult
+outputStrict x | x == x = output x
+               | otherwise = fail "I am the pope."
+
+checkCategory :: PGF -> Category -> CGI ()
+checkCategory pgf cat = unless (cat `elem` categories pgf) $ 
+                        throwCGIError 400 "Unknown category" ["Unknown category: " ++ cat]
+
+checkLanguage :: PGF -> Category -> CGI ()
+checkLanguage pgf lang = unless (lang `elem` languages pgf) $ 
+                         throwCGIError 400 "Unknown language" ["Unknown language: " ++ lang]
 
 translate :: PGF -> String -> Maybe Category -> Maybe Language -> Maybe Language -> Translation
 translate pgf input mcat mfrom mto = 
-    Record [(from, [Record [(to, linearize pgf to tree) | to <- toLangs] | tree <- parse pgf from cat input]) 
-                | from <- fromLangs]
+    Record [(from, [Record [(to, linearize pgf to tree) | to <- toLangs] | tree <- trees]) 
+                | from <- fromLangs, let trees = parse pgf from cat input, not (null trees)]
   where cat       = fromMaybe (startCat pgf) mcat
         fromLangs = maybe (languages pgf) (:[]) mfrom
-        toLangs   = maybe (languages pgf) (:[]) mfrom
-            
\ No newline at end of file
+        toLangs   = maybe (languages pgf) (:[]) mto
+
+
+-- * General CGI Error exception mechanism
+
+data CGIError = CGIError { cgiErrorCode :: Int, cgiErrorMessage :: String, cgiErrorText :: [String] }
+                deriving Typeable
+
+throwCGIError :: Int -> String -> [String] -> CGI a
+throwCGIError c m t = throwCGI $ DynException $ toDyn $ CGIError c m t
+
+handleCGIErrors :: CGI CGIResult -> CGI CGIResult
+handleCGIErrors x = x `catchCGI` \e -> case e of
+                                         DynException d -> case fromDynamic d of
+                                                             Nothing -> throw e
+                                                             Just (CGIError c m t) -> outputError c m t
+                                         _ -> throw e