diff --git a/lib/tmp.js b/lib/tmp.js
index 5859d74..46d88cf 100644
--- a/lib/tmp.js
+++ b/lib/tmp.js
@@ -145,6 +145,9 @@ function _generateTmpName(opts) {
     opts.postfix || ''
   ].join('');
 
+  if (!opts.dir && !tmpDir)
+    throw new Error("no tmp dir specified");
+
   return path.join(opts.dir || tmpDir, name);
 }