diff --git a/sourcemap.cabal b/sourcemap.cabal
--- a/sourcemap.cabal
+++ b/sourcemap.cabal
@@ -1,5 +1,5 @@
 name:                sourcemap
-version:             0.1.0.3
+version:             0.1.1.0
 synopsis:            Implementation of source maps as proposed by Google and Mozilla.
 description:         Implementation of source maps, revision 3, proposed by Google and Mozilla here
                      <https://wiki.mozilla.org/DevTools/Features/SourceMap> and here
@@ -20,7 +20,7 @@
   other-modules:       VLQ
   hs-source-dirs:      src
   build-depends:       base >= 4 && < 5,
-                       bytestring >= 0.10.2.0,
+                       bytestring,
                        aeson,
                        unordered-containers,
                        attoparsec,
diff --git a/src/SourceMap.hs b/src/SourceMap.hs
--- a/src/SourceMap.hs
+++ b/src/SourceMap.hs
@@ -14,6 +14,7 @@
 import           Data.Aeson hiding ((.=))
 import           Data.ByteString.Lazy (ByteString)
 import qualified Data.ByteString.Lazy as Bytes
+import           Data.ByteString.Lazy.UTF8 (fromString)
 import           Data.Foldable (forM_)
 import qualified Data.HashMap.Lazy as Map
 import           Data.List
@@ -57,7 +58,7 @@
                                              (fromIntegral (fromEnum ';'))
                    return (posLine mapGenerated)
            else do when (i > 0)
-                        (result += ",")
+                        (result += (fromString ","))
                    return previousGeneratedLine
       -- Original generated column (also offsetted from previous entries).
       updating prevGenCol $ \previousGeneratedColumn -> do