Missing value: load15

I have load15, disk and ram similar alerts (differences are only in measurement, value name and warn/crit). All alerts work fine (including load15) but I am getting this error in logs
```
ts=2018-01-18T10:10:37.069Z lvl=error msg="error evaluating expression" service=kapacitor task_master=main task=load15_alert node=eval2 err="missing value: \"load15\""
```
My config
```
/etc/kapacitor/load/tasks/load15_alert.tick
dbrp "telegraf"."autogen"

var warn = 1
var crit = 2
var whereFilter = lambda: TRUE

var data = stream
|from()
.measurement('system')
.groupBy('host')
.where(whereFilter)
|eval(lambda: "load15")
.as('value')

var trigger = data
|alert()
.id('{{ index .Tags "host"}}/load15')
.crit(lambda: "value" > crit)
.warn(lambda: "value" > warn)
.stateChangesOnly()
.message('{{ .Level }} - {{ index .Tags "host" }} load15 is {{ index .Fields "value" }}')
.post('http://example.com/api/alert')
.slack()
```
When I disable this task, error disappears.
Debian 9
Kapacitor 1.4.0

Use /etc/kapacitor/load/tasks/load15_alert.tick and the reported Kapacitor 1.4.0 configuration as the reproduction case. Start by tracing the eval2 expression and the system measurement fields involved in the load15 task; done means explaining and correcting the missing-value error without disabling the alert.

Written by the indexing model from the issue text.