diff --git a/.github/workflows/build.sh b/.github/workflows/build.sh
index 0d4afab3..5dfa741e 100755
--- a/.github/workflows/build.sh
+++ b/.github/workflows/build.sh
@@ -17,6 +17,7 @@ fi
 
 export CC=$GH_COMPILER
 export CXX=${CC%cc}++
+export CXXFLAGS_EARLY=-Werror
 
 if [[ "$GH_BUILDSYS" == "makefile" ]]; then
   make
diff --git a/.travis.yml b/.travis.yml
index 95516a05..6c8e9887 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -162,6 +162,7 @@ script:
   fi
 - |-
   # Build hyperrogue.
+  export CXXFLAGS_EARLY=-Werror
   if [[ "$TRAVIS_BUILD_SYSTEM" == "Makefile" ]]; then
     make
   elif [[ "$TRAVIS_BUILD_SYSTEM" == "mymake" ]]; then