diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1a1c429..d3aa8d5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## 0.16.0 (2022-xx-xx)
 
+### Changes
+
+- Fix labels cardinality error when registering unknown pre-auth key [#519](https://github.com/juanfont/headscale/pull/519)
+
 ## 0.15.0 (2022-03-20)
 
 **Note:** Take a backup of your database before upgrading.
diff --git a/api.go b/api.go
index eab8076..61ec1b5 100644
--- a/api.go
+++ b/api.go
@@ -573,7 +573,7 @@ func (h *Headscale) handleAuthKey(
 			machineRegistrations.WithLabelValues("new", RegisterMethodAuthKey, "error", pak.Namespace.Name).
 				Inc()
 		} else {
-			machineRegistrations.WithLabelValues("new", RegisterMethodAuthKey, "error").Inc()
+			machineRegistrations.WithLabelValues("new", RegisterMethodAuthKey, "error", "unknown").Inc()
 		}
 
 		return