diff --git a/liblastfm.cabal b/liblastfm.cabal
--- a/liblastfm.cabal
+++ b/liblastfm.cabal
@@ -1,5 +1,5 @@
 name: liblastfm
-version: 0.0.3.5
+version: 0.0.3.6
 synopsis: Wrapper to Lastfm API
 license: MIT
 license-file: LICENSE
@@ -99,7 +99,7 @@
 
 test-suite json-authentication
   build-depends: base >= 3 && < 5,
-                 bytestring == 0.9.*,
+                 bytestring >= 0.9 && < 0.11,
                  aeson == 0.6.*,
                  HUnit == 1.2.4.*,
                  liblastfm
diff --git a/src/Network/Lastfm/Internal.hs b/src/Network/Lastfm/Internal.hs
--- a/src/Network/Lastfm/Internal.hs
+++ b/src/Network/Lastfm/Internal.hs
@@ -23,20 +23,19 @@
 
 
 -- Response format
-data Format = Format { errorParser ∷ Response → Maybe LastfmError, uriArgument ∷ (String, String) }
+data Format = Format { errorParser ∷ Response → Maybe LastfmError, uriArgument ∷ Maybe (String, String) }
 
 
 -- Send POST query to Lastfm API
 callAPI ∷ Format → [(String, String)] → Lastfm Response
-callAPI Format {errorParser = e, uriArgument = arg} = query e . (arg:) . map (second encodeString)
-
+callAPI Format {errorParser = e, uriArgument = arg} = query e . (maybe id (:) arg) . map (second encodeString)
 
 -- Send signed POST query to Lastfm API
 callAPIsigned ∷ Format → Secret → [(String, String)] → Lastfm Response
 callAPIsigned Format {errorParser = e, uriArgument = arg} (Secret s) xs = query e zs
  where
   ys = map (second encodeString) . filter (not . null . snd) $ xs
-  zs = (arg :) $ ("api_sig", sign ys) : ys
+  zs = maybe id (:) arg $ ("api_sig", sign ys) : ys
 
   sign ∷ [(String, String)] → String
   sign = show . md5 . BS.pack . (++ s) . concatMap (uncurry (++)) . sortBy (compare `on` fst)
diff --git a/src/Network/Lastfm/JSON.hs b/src/Network/Lastfm/JSON.hs
--- a/src/Network/Lastfm/JSON.hs
+++ b/src/Network/Lastfm/JSON.hs
@@ -17,7 +17,7 @@
 json ∷ Format
 json = Format
         { errorParser = decode
-        , uriArgument = ("format","json")
+        , uriArgument = Just ("format","json")
         }
 
 
diff --git a/src/Network/Lastfm/XML.hs b/src/Network/Lastfm/XML.hs
--- a/src/Network/Lastfm/XML.hs
+++ b/src/Network/Lastfm/XML.hs
@@ -18,7 +18,7 @@
 xml ∷ Format
 xml = Format
         { errorParser = xmlErrorParser
-        , uriArgument = ("format","xml")
+        , uriArgument = Nothing
         }