diff --git a/src/com/mirantis/mk/openstack.groovy b/src/com/mirantis/mk/openstack.groovy
index 6702be6..ec840d8 100644
--- a/src/com/mirantis/mk/openstack.groovy
+++ b/src/com/mirantis/mk/openstack.groovy
@@ -212,7 +212,7 @@
     cmd = "heat output-show ${name} ${outputParam}"
     output = runOpenstackCommand(cmd, env, path)
     echo("${cmd}: ${output}")
-    return output.substring(1, output.length()-1)
+    return "${output}".substring(1, output.length()-1)
 }
 
 /**