Error found when convert int to float

This is the error found in /var/log/kapacitor/kapacitor.log:

```
[system_load:eval2] 2017/06/15 13:55:29 E! Cannot call function "float" with args signature (missing), available signatures are [(boolean), (string), (int), (float)].
```

And this is the system_load tickscript:

```stream
|from()
.database('telegraf')
.retentionPolicy('autogen')
.measurement('system')
.groupBy('host', 'inst')
|eval(lambda: "load1" / float("n_cpus"))
.as('perload')
|alert()
.id('{{ index .Tags "host" }}/{{ index .Tags "inst" }}')
.message('{{ .Level}}: {{ .ID }} has high perload: {{ index .Fields "perload" }}')
.stateChangesOnly()
.crit(lambda: "perload" > 0.5)
.exec('/data/mysqlscript/tick/alarm.py')```

This error continue to appear in error log, but the question is, the alert still be triggered correctly...
So what does the error mean?

Start with the reported Kapacitor log entry and the tickscript expression `float("n_cpus")`; trace how the eval expression handles the field and why it reports a missing argument. Compare that behavior with the alert result shown in the report. Done means documenting the cause of the error and whether it affects alert correctness.

Written by the indexing model from the issue text.