diff --git a/lib/rsa-ursa.js b/lib/rsa-ursa.js
index 8c9722b..725e76c 100644
--- a/lib/rsa-ursa.js
+++ b/lib/rsa-ursa.js
@@ -1,6 +1,6 @@
 'use strict';
 
-var ursa = require('ursa');
+var ursa = require('ursa-optional');
 
 function notToJson() {
   return undefined;
diff --git a/package.json b/package.json
index cbd2e78..db253cb 100644
--- a/package.json
+++ b/package.json
@@ -39,7 +39,8 @@
   },
   "homepage": "https://git.coolaj86.com/coolaj86/rsa-compat.js#readme",
   "dependencies": {
-    "node-forge": "^0.6.41"
+    "node-forge": "^0.6.41",
+    "ursa-optional": "^0.9.4"
   },
   "trulyOptionalDependencies": {
     "buffer-v6-polyfill": "^1.0.3",