in my case it was:

tolerations:
- operator: Exists # matches all keys, values and effects which means this will tolerate everything

An empty key with operator Exists matches all keys, values and effects which means this will tolerate everything.

tolerations:
- operator: "Exists"

From: https://kubernetes.io/docs/concepts/configuration/taint-and-toleration/#concepts

If you want to run a daemonset and make sure it will get scheduled onto all nodes in the cluster regardless of taints. For example in a GKE cluster running google’s Stackdriver logging agent the fluentd-gcp daemonset has the following toleration to make sure it gets past any node taint:

tolerations:
-operator: Exists
 effect: NoExecute
-operator: Exists
 effect: NoSchedule

This way you can scheduler the daemonset on the master even if it has NoSchedule taints.