diff --git a/examples/readme/Main.hs b/examples/readme/Main.hs
--- a/examples/readme/Main.hs
+++ b/examples/readme/Main.hs
@@ -2,10 +2,10 @@
 
 import Control.Concurrent
 import Control.Concurrent.Async
-import LightStep.HighLevel.IO (LogEntryKey (..), addLog, getEnvConfig, setTag, withSingletonLightStep, withSpan)
+import LightStep.HighLevel.IO (LogEntryKey (..), addLog, currentSpanContext, getEnvConfig, setParentSpanContext, setTag, withSingletonLightStep, withSpan)
 
 seriousBusinessMain :: IO ()
-seriousBusinessMain = concurrently_ frontend backend
+seriousBusinessMain = concurrently_ frontend backend >> threadDelay 1000000
   where
     frontend =
       withSpan "RESTful API" $ do
@@ -43,6 +43,25 @@
         withSpan "Hadoop" $ do
           threadDelay 100000
           setTag "learning" "super_deep"
+        withSpan "Parallel map reduce" $ do
+          result <- withSpan "Reduce" $ do
+            Just ctx <- currentSpanContext
+            (a, b) <- do
+              threadDelay 100000
+              concurrently
+                ( withSpan "Calculate a" $ do
+                    setParentSpanContext ctx
+                    threadDelay 100000
+                    return "Lorem "
+                )
+                ( withSpan "Calculate b" $ do
+                    setParentSpanContext ctx
+                    threadDelay 100000
+                    return "ipsum"
+                )
+            threadDelay 100000
+            pure (a <> b)
+          setTag "result" result
 
 main :: IO ()
 main = do
diff --git a/lightstep-haskell.cabal b/lightstep-haskell.cabal
--- a/lightstep-haskell.cabal
+++ b/lightstep-haskell.cabal
@@ -1,7 +1,7 @@
 cabal-version: 2.4
 
 name:           lightstep-haskell
-version:        0.5.1
+version:        0.5.2
 synopsis:       LightStep OpenTracing client library
 description:    LightStep OpenTracing client library. Uses GRPC transport via proto-lens.
 category:       Tools
diff --git a/src/LightStep/HighLevel/IO.hs b/src/LightStep/HighLevel/IO.hs
--- a/src/LightStep/HighLevel/IO.hs
+++ b/src/LightStep/HighLevel/IO.hs
@@ -89,10 +89,9 @@
   tId <- myThreadId
   modifyMVar_
     globalSharedMutableSpanStacks
-    ( \stacks ->
-        let (sp : sps) = stacks HM.! tId
-            !stacks' = HM.insert tId (f sp : sps) stacks
-         in pure stacks'
+    ( \stacks -> case HM.lookup tId stacks of
+        Just (sp : sps) -> pure $! HM.insert tId (f sp : sps) stacks
+        _ -> pure stacks
     )
 
 currentSpanContext :: MonadIO m => m (Maybe SpanContext)
@@ -100,8 +99,8 @@
   tId <- myThreadId
   stacks <- readMVar globalSharedMutableSpanStacks
   let ctx =
-        case stacks HM.! tId of
-          (sp : _) -> Just $ sp ^. spanContext
+        case HM.lookup tId stacks of
+          Just (sp : _) -> Just $ sp ^. spanContext
           _ -> Nothing
   pure ctx