diff --git a/build-scripts/postfix-install.sh b/build-scripts/postfix-install.sh
index d9e9911..bf587df 100644
--- a/build-scripts/postfix-install.sh
+++ b/build-scripts/postfix-install.sh
@@ -24,7 +24,7 @@ do_ubuntu() {
     apt-get install -y postfix postfix-pcre postfix-ldap
     apt-get install -y opendkim
     local libcurl="libcurl4"
-    if "$(apt-cache search --names-only '^libcurl4t64$')" != ""; then
+    if [ "$(apt-cache search --names-only '^libcurl4t64$')" != "" ]; then
         libcurl="libcurl4t64"
     fi
     apt-get install -y ca-certificates tzdata supervisor rsyslog bash opendkim-tools curl ${libcurl} libjsoncpp25 sasl2-bin postfix-lmdb procps logrotate cron net-tools colorized-logs netcat-openbsd ${RELEASE_SPECIFIC_PACKAGES}