diff --git a/lib/heat b/lib/heat
index 3e6975a..c22369f 100644
--- a/lib/heat
+++ b/lib/heat
@@ -59,10 +59,10 @@
 # other default options
 if [[ "$HEAT_STANDALONE" = "True" ]]; then
     # for standalone, use defaults which require no service user
-    HEAT_STACK_DOMAIN=`trueorfalse False $HEAT_STACK_DOMAIN`
+    HEAT_STACK_DOMAIN=$(trueorfalse False HEAT_STACK_DOMAIN)
     HEAT_DEFERRED_AUTH=${HEAT_DEFERRED_AUTH:-password}
 else
-    HEAT_STACK_DOMAIN=`trueorfalse True $HEAT_STACK_DOMAIN`
+    HEAT_STACK_DOMAIN=$(trueorfalse True HEAT_STACK_DOMAIN)
     HEAT_DEFERRED_AUTH=${HEAT_DEFERRED_AUTH:-trusts}
 fi