diff --git a/lib/parse-url.js b/lib/parse-url.js
index 7d5489c..bfd54b9 100644
--- a/lib/parse-url.js
+++ b/lib/parse-url.js
@@ -21,20 +21,23 @@ const correctProtocol = (arg, protocols) => {
     return arg
   }
 
+  if (arg.substr(firstColon, 3) === '://') {
+    // If arg is given as <foo>://<bar>, then this is already a valid URL.
+    return arg
+  }
+
   const firstAt = arg.indexOf('@')
   if (firstAt > -1) {
     if (firstAt > firstColon) {
+      // URL has the form of <foo>:<bar>@<baz>. Assume this is a git+ssh URL.
       return `git+ssh://${arg}`
     } else {
+      // URL has the form 'git@github.com:npm/hosted-git-info.git'.
       return arg
     }
   }
 
-  const doubleSlash = arg.indexOf('//')
-  if (doubleSlash === firstColon + 1) {
-    return arg
-  }
-
+  // Correct <foo>:<bar> to <foo>://<bar>
   return `${arg.slice(0, firstColon + 1)}//${arg.slice(firstColon + 1)}`
 }
 
diff --git a/test/parse-url.js b/test/parse-url.js
index 54e7d16..70f04b1 100644
--- a/test/parse-url.js
+++ b/test/parse-url.js
@@ -15,3 +15,10 @@ t.test('can parse file urls', async t => {
   t.ok(parseUrl(u))
   t.ok(HostedGit.parseUrl(u))
 })
+
+t.test('can parse custom urls', async t => {
+  const u = 'foobar://user:host@path'
+  t.ok(parseUrl(u, {}))
+  t.equal(parseUrl(u, {}).protocol, 'foobar:')
+  t.ok(HostedGit.parseUrl(u))
+})