diff --git a/Cabal/Cabal.cabal b/Cabal/Cabal.cabal
index 711f4dea636..4bd11d9b040 100644
--- a/Cabal/Cabal.cabal
+++ b/Cabal/Cabal.cabal
@@ -83,6 +83,7 @@ extra-source-files:
   tests/PackageTests/Haddock/CPP.hs
   tests/PackageTests/Haddock/Literate.lhs
   tests/PackageTests/Haddock/my.cabal
+  tests/PackageTests/Haddock/NoCPP.hs
   tests/PackageTests/Haddock/Simple.hs
   tests/PackageTests/OrderFlags/Foo.hs
   tests/PackageTests/OrderFlags/my.cabal
diff --git a/Cabal/Distribution/Simple/Haddock.hs b/Cabal/Distribution/Simple/Haddock.hs
index 79c7d050541..7e1b10a72b7 100644
--- a/Cabal/Distribution/Simple/Haddock.hs
+++ b/Cabal/Distribution/Simple/Haddock.hs
@@ -27,7 +27,7 @@ import Distribution.Package
          , PackageName(..), packageName )
 import qualified Distribution.ModuleName as ModuleName
 import Distribution.PackageDescription as PD
-         ( PackageDescription(..), BuildInfo(..), allExtensions
+         ( PackageDescription(..), BuildInfo(..), usedExtensions
          , Library(..), hasLibs, Executable(..)
          , TestSuite(..), TestSuiteInterface(..)
          , Benchmark(..), BenchmarkInterface(..) )
@@ -365,7 +365,7 @@ getGhcCppOpts haddockVersion bi =
         ghcOptCppOptions   = defines
     }
   where
-    needsCpp             = EnableExtension CPP `elem` allExtensions bi
+    needsCpp             = EnableExtension CPP `elem` usedExtensions bi
     defines              = [haddockVersionMacro]
     haddockVersionMacro  = "-D__HADDOCK_VERSION__="
                            ++ show (v1 * 1000 + v2 * 10 + v3)
diff --git a/Cabal/tests/PackageTests/Haddock/Check.hs b/Cabal/tests/PackageTests/Haddock/Check.hs
index cbfe522c078..0d41dd45156 100644
--- a/Cabal/tests/PackageTests/Haddock/Check.hs
+++ b/Cabal/tests/PackageTests/Haddock/Check.hs
@@ -26,7 +26,7 @@ suite ghcPath = TestCase $ do
     assertHaddockSucceeded hResult
 
     let docFiles = map (haddocksDir </>)
-                       ["CPP.html", "Literate.html", "Simple.html"]
+                       ["CPP.html", "Literate.html", "NoCPP.html", "Simple.html"]
     mapM_ (assertFindInFile "For hiding needles.") docFiles
 
 assertFindInFile :: String -> FilePath -> Assertion
diff --git a/Cabal/tests/PackageTests/Haddock/NoCPP.hs b/Cabal/tests/PackageTests/Haddock/NoCPP.hs
new file mode 100644
index 00000000000..417f368b37c
--- /dev/null
+++ b/Cabal/tests/PackageTests/Haddock/NoCPP.hs
@@ -0,0 +1,8 @@
+module NoCPP (Haystack) where
+
+-- | For hiding needles.
+data Haystack = Haystack
+
+-- | Causes a build failure if the CPP language extension is enabled.
+stringGap = "Foo\
+\Bar"
diff --git a/Cabal/tests/PackageTests/Haddock/my.cabal b/Cabal/tests/PackageTests/Haddock/my.cabal
index 63e9faa4a7c..176ced9b4de 100644
--- a/Cabal/tests/PackageTests/Haddock/my.cabal
+++ b/Cabal/tests/PackageTests/Haddock/my.cabal
@@ -11,6 +11,6 @@ description:
     Check that Cabal successfully invokes Haddock.
 
 Library
-    exposed-modules: CPP, Literate, Simple
+    exposed-modules: CPP, Literate, NoCPP, Simple
     other-extensions: CPP
     build-depends: base