Message variable from a rule returns NULL in the email notification (61844)

Configuring rules to send emails; the "mail.message" section is configured as:

"[Critical] @Message The following URL can be used to obtain alarm details. @foglight_rule_alarm_link"

When the emails are delivered some work correctly - for example:

[Critical] Virtual machine my_server is using excessive ( 100%) of the CPU processing power available to it. This value is outside the normal range of CPU utilization. The following URL can be used to obtain alarm details. http://my_FMS:8080/console/page/view/system:core_alarms.27?AlarmID=/alarms/610a6adf-ff77-4ae5-98f9-2aa5f11368a8

and some do not - for example:

[Critical] null The following URL can be used to obtain alarm details. http://my_FMS:8080/console/page/view/system:core_alarms.27?AlarmID=/alarms/dff3610f-df96-4b97-abdb-df556daeab0e

Message variable is not defined in the rule definition

Please make sure that the message variable is defined in the rule definition.
Open the rule, click on the "Rule Variable" tab and check the variable definition there.

As an example:
The rule "VMW Virtual Machine CPU Utilization" defines the following message variable:
---
message = "Virtual machine ";
anyConditions = false;
message += scope.get("name");
if(#utilization from $scope.hostCPUs #>registry("VMW:virtualMachineCPU.warning"))

message+=" is using excessive ( "+Math.round(1.0*# utilization from $scope.hostCPUs #)+ "%) of the CPU processing power available to it. This value";
anyConditions = true;

history = #utilization from $scope.hostCPUs for 90 min#;
mean = avg(history);
std = stddev(history);
val = Math.abs(#utilization from $scope.hostCPUs #-mean);
if((val/std) < registry("VMW:cpu.STDev.warning"))
{message+=" is within the normal range of CPU utilization.";}
else
{message+=" is outside the normal range of CPU utilization.";}

return message;
---

But if you look the rule "VMW Virtual Machine Network IO" no message variable is defined.