Can't evaluate field existingSecret in type interface {}

Hello,

I am trying to use helm to install stable/grafana by values.yaml
github@com/ helm/charts/blob/master/ stable/grafana/values.yaml# L363

the vlues.yaml edited the following smtp option

smtp:

existingSecret is a reference to an existing secret containing the smtp configuration

for Grafana.

enabled = true
host = smtp .gmail.com:587
user = xxx@gmail.com
password = xxxxx
#cert_file =
#key_file =
skip_verify = true
from_address = xxx@gmail.com

But showing error when installing:

[root@sealos01 grafana]# helm install -f values. yaml --name test stable/grafana
Error: render error in “grafana/templates/deployment. yaml”: template: grafana/templates/deployment.yaml:46:10: executing “grafana/templates/deployment. yaml” at <include “grafana. pod” .>: error calling include: template: grafana/templates/_pod.tpl:298:20: executing “grafana. pod” at <.Values. smtp .existingSecret>: can’t evaluate field existingSecret in type interface {}

Thanks