diff --git a/src/Monitor/Tracing/Zipkin.hs b/src/Monitor/Tracing/Zipkin.hs
--- a/src/Monitor/Tracing/Zipkin.hs
+++ b/src/Monitor/Tracing/Zipkin.hs
@@ -131,7 +131,9 @@
       { HTTP.method = "POST"
       , HTTP.host = BS.pack (fromMaybe "localhost" mbHostname)
       , HTTP.path = "/api/v2/spans"
-      , HTTP.port = maybe 9411 fromIntegral mbPort }
+      , HTTP.port = maybe 9411 fromIntegral mbPort
+      , HTTP.requestHeaders = [("content-type", "application/json")]
+      }
   void $ let prd = fromMaybe 0 mbPrd in if prd <= 0
     then pure Nothing
     else fmap Just $ forkIO $ forever $ do
diff --git a/tracing.cabal b/tracing.cabal
--- a/tracing.cabal
+++ b/tracing.cabal
@@ -1,7 +1,7 @@
 cabal-version: 1.12
 
 name: tracing
-version: 0.0.5.1
+version: 0.0.5.2
 synopsis: Distributed tracing
 description:
   An OpenTracing-compliant, simple, and extensible distributed tracing library.
@@ -34,7 +34,7 @@
   build-depends:
       aeson >= 1.4
     , base >= 4.9 && < 5
-    , base16-bytestring >= 0.1
+    , base16-bytestring >= 0.1 && < 1.0.0.0
     , bytestring >= 0.10
     , case-insensitive >= 1.2
     , containers >= 0.6