diff --git a/acme/autocert/autocert.go b/acme/autocert/autocert.go
index efc482cc..cd613534 100644
--- a/acme/autocert/autocert.go
+++ b/acme/autocert/autocert.go
@@ -276,7 +276,7 @@ func supportsECDSA(hello *tls.ClientHelloInfo) bool {
 		ecdsaOK := false
 	schemeLoop:
 		for _, scheme := range hello.SignatureSchemes {
-			var tlsECDSAWithSHA1 tls.SignatureScheme = 0x0203 // constant added in Go 1.10
+			const tlsECDSAWithSHA1 tls.SignatureScheme = 0x0203 // constant added in Go 1.10
 			switch scheme {
 			case tlsECDSAWithSHA1, tls.ECDSAWithP256AndSHA256,
 				tls.ECDSAWithP384AndSHA384, tls.ECDSAWithP521AndSHA512: