diff --git a/ci.sh b/ci.sh
index 8c3fcebe94d..785d7aa5520 100755
--- a/ci.sh
+++ b/ci.sh
@@ -48,4 +48,7 @@ elif [ "${TRAVIS_OS_NAME:-}" == osx ]; then
 elif [ "${CI_WINDOWS:-}" == True ]; then
   MIRI_TEST_TARGET=x86_64-unknown-linux-gnu run_tests
   MIRI_TEST_TARGET=x86_64-apple-darwin run_tests
+else
+  echo "FATAL: unknown CI platform"
+  exit 1
 fi