diff --git a/.travis.yml b/.travis.yml
index 32351fd72..ff8ad8be9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -13,7 +13,7 @@ before_install:
     git clone -q -n "https://github.com/${TRAVIS_REPO_SLUG}.git" "${TRAVIS_REPO_SLUG}"
     cd -- "${TRAVIS_REPO_SLUG}"
     to_fetch=("${TRAVIS_COMMIT}")
-    if [ -n "${TRAVIS_PULL_REQUEST-}" ]; then to_fetch+=("+refs/pull/${TRAVIS_PULL_REQUEST}/merge:"); fi
+    if [ false != "${TRAVIS_PULL_REQUEST-}" ]; then to_fetch+=("+refs/pull/${TRAVIS_PULL_REQUEST}/merge:"); fi
     git fetch -q -- origin "${to_fetch[@]}"
     git checkout -qf "${TRAVIS_COMMIT}" --