diff --git a/hooks/build b/hooks/build
index e04b4c2..6f54521 100755
--- a/hooks/build
+++ b/hooks/build
@@ -8,7 +8,7 @@ set -ex
 . "hooks/config"
 
 for DOCKER_IMAGE in ${DOCKER_IMAGES}; do
-  if [ ${TRAVIS} == 'true' ]; then
+  if [ "${TRAVIS}" == 'true' ]; then
     echo "Build Zammad Docker image ${DOCKER_IMAGE} with version ${TRAVIS_BUILD_NUMBER} for DockerHubs ${DOCKER_REGISTRY}/${GITHUB_USERNAME}/${DOCKER_REPOSITORY}:${DOCKER_IMAGE} repo"
 
     docker build --pull --no-cache --build-arg BUILD_DATE=$(date -u +”%Y-%m-%dT%H:%M:%SZ”) -t ${DOCKER_REGISTRY}/${GITHUB_USERNAME}/${DOCKER_REPOSITORY}:${DOCKER_IMAGE} -t ${DOCKER_REGISTRY}/${GITHUB_USERNAME}/${DOCKER_REPOSITORY}:${DOCKER_IMAGE}-${TRAVIS_BUILD_NUMBER} -f containers/${DOCKER_IMAGE}/Dockerfile .