diff --git a/.travis.yml b/.travis.yml
index 58a0f33..dbe5dfc 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -250,6 +250,8 @@ install:
       cabal --version
       travis_retry cabal update -v
       echo 'packages: .' > cabal.project
+      echo 'package brittany'        > cabal.project.local
+      echo '  ghc-options: -Werror' >> cabal.project.local
       rm -f cabal.project.freeze
       cabal new-build -j$JOBS --enable-test --enable-benchmarks --dep
       cabal new-build -j$JOBS --disable-tests --disable-benchmarks --dep
@@ -262,12 +264,12 @@ script:
   set -ex
   case "$BUILD" in
     stack)
-      better_wait stack --no-terminal $ARGS test --bench --no-run-benchmarks --haddock --no-haddock-deps --ghc-options="-j1 +RTS -M500M -RTS"
+      better_wait stack --no-terminal $ARGS test --bench --no-run-benchmarks --haddock --no-haddock-deps --ghc-options="-j1 +RTS -M500M -RTS -Werror"
       ;;
     cabal)
       if [ -f configure.ac ]; then autoreconf -i; fi
       cabal configure --enable-tests --enable-benchmarks -v  # -v2 provides useful information for debugging
-      better_wait cabal build -j$JOBS --ghc-options="-j1 +RTS -M500M -RTS" # this builds all libraries and executables (including tests/benchmarks)
+      better_wait cabal build -j$JOBS --ghc-options="-j1 +RTS -M500M -RTS -Werror" # this builds all libraries and executables (including tests/benchmarks)
       cabal test
       ;;
     cabaldist)