Fixes #185.

lib/core/package.js

@@ -540,6 +540,10 @@ Package.prototype.checkout = function () {
 
     // If tag is specified, try to satisfy it
     if (this.tag) {
+      if (!semver.valid(this.tag)) {
+        return this.emit('error', new Error('Tag ' + this.tag + ' is not a valid semver version'));
+      }
+
       versions = versions.filter(function (version) {
         return semver.satisfies(version, this.tag);
       }.bind(this));