diff --git a/README.md b/README.md
index 2d175ea..2a28867 100644
--- a/README.md
+++ b/README.md
@@ -28,4 +28,9 @@ assert(satisfies(
 assert(satisfies('(MIT OR GPL-2.0)', '(ISC OR MIT)'))
 assert(satisfies('(MIT AND GPL-2.0)', '(MIT OR GPL-2.0)'))
 assert(!satisfies('(MIT AND GPL-2.0)', '(ISC OR GPL-2.0)'))
+
+// bring your own parser
+satisfies('MIT', 'MIT', {
+  parse: function (expression) { return {license: 'MIT'} }
+})
 ```
diff --git a/index.js b/index.js
index 2eb22a6..2f8f352 100644
--- a/index.js
+++ b/index.js
@@ -93,7 +93,7 @@ function normalizeGPLIdentifiers (argument) {
       argument.license = license.replace('-or-later', '')
       delete argument.plus
     }
-  } else {
+  } else if (argument.left && argument.right) {
     argument.left = normalizeGPLIdentifiers(argument.left)
     argument.right = normalizeGPLIdentifiers(argument.right)
   }
@@ -104,7 +104,9 @@ function endsWith (string, substring) {
   return string.indexOf(substring) === string.length - 1
 }
 
-module.exports = function (first, second) {
+module.exports = function (first, second, options) {
+  options = options || {}
+  parse = options.parse || parse
   return recurse(
     normalizeGPLIdentifiers(parse(first)),
     normalizeGPLIdentifiers(parse(second))