diff --git a/app/cmd/config.go b/app/cmd/config.go
index 0e1d410..65f7e4e 100644
--- a/app/cmd/config.go
+++ b/app/cmd/config.go
@@ -104,6 +104,9 @@ func (c *serverConfig) Check() error {
 	if len(c.ACME.Domains) == 0 && (len(c.CertFile) == 0 || len(c.KeyFile) == 0) {
 		return errors.New("need either ACME info or cert/key files")
 	}
+	if len(c.ACME.Domains) > 0 && (len(c.CertFile) > 0 || len(c.KeyFile) > 0) {
+		return errors.New("cannot use both ACME and cert/key files, they are mutually exclusive")
+	}
 	if up, down, err := c.Speed(); err != nil || (up != 0 && up < minSpeedBPS) || (down != 0 && down < minSpeedBPS) {
 		return errors.New("invalid speed")
 	}