diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+1.5.2
+
+* Increase upper bound on `async`
+
 1.5.1
 
 * GHC 8.4 support
diff --git a/src/Turtle/Prelude.hs b/src/Turtle/Prelude.hs
--- a/src/Turtle/Prelude.hs
+++ b/src/Turtle/Prelude.hs
@@ -1976,11 +1976,11 @@
 
         done' (Pair x _) = done x
 
--- | Returns the result of a 'Shell' that outputs a single
--- line:
+-- | Returns the result of a 'Shell' that outputs a single line.
+-- Note that if no lines / more than 1 line is produced by the Shell, this function will `die` with an error message.
 --
 -- > main = do
--- >   Just directory <- single (inshell "pwd" empty)
+-- >   directory <- single (inshell "pwd" empty)
 -- >   print directory
 single :: MonadIO io => Shell a -> io a
 single s = do
diff --git a/turtle.cabal b/turtle.cabal
--- a/turtle.cabal
+++ b/turtle.cabal
@@ -1,5 +1,5 @@
 Name: turtle
-Version: 1.5.1
+Version: 1.5.2
 Cabal-Version: >=1.10
 Build-Type: Simple
 License: BSD3
@@ -51,7 +51,7 @@
     Build-Depends:
         base                 >= 4.6     && < 5   ,
         ansi-wl-pprint       >= 0.6     && < 0.7 ,
-        async                >= 2.0.0.0 && < 2.2 ,
+        async                >= 2.0.0.0 && < 2.3 ,
         bytestring           >= 0.9.1.8 && < 0.11,
         clock                >= 0.4.1.2 && < 0.8 ,
         containers           >= 0.5.0.0 && < 0.6 ,