diff --git a/ci/before_script.sh b/ci/before_script.sh
index 8bab1c4e17..701fe1d9eb 100755
--- a/ci/before_script.sh
+++ b/ci/before_script.sh
@@ -7,7 +7,7 @@ CI_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 source "${CI_DIR}/common/build.sh"
 
 # Enable ipv6 on Travis. ref: a39c8b7ce30d
-if ! test "${TRAVIS_OS_NAME}" = osx ; then
+if test -n "${TRAVIS_OS_NAME}" && ! test "${TRAVIS_OS_NAME}" = osx ; then
   echo "before_script.sh: enable ipv6"
   sudo sysctl -w net.ipv6.conf.lo.disable_ipv6=0
 fi