diff --git a/lib/help/key_object.js b/lib/help/key_object.js
index 24ed3bdec9..b1d3bcde5d 100644
--- a/lib/help/key_object.js
+++ b/lib/help/key_object.js
@@ -274,7 +274,7 @@ if (!keyObjectSupported) {
           label = 'RSA PUBLIC KEY'
           break
         case '-----BEGIN CERTIFICATE-----':
-          throw new errors.JOSENotSupported('X.509 certificates are supported in your Node.js runtime version')
+          throw new errors.JOSENotSupported('X.509 certificates are not supported in your Node.js runtime version')
         default:
           throw new TypeError('unknown/unsupported PEM type')
       }