diff --git a/index.js b/index.js
index e1384a2..396c5ba 100644
--- a/index.js
+++ b/index.js
@@ -31,7 +31,7 @@ const UNIT_MAP = {
   h: ['h', 'hr', 'hrs', 'hour', 'hours'],
   d: ['d', 'day', 'days'],
   w: ['w', 'week', 'weeks'],
-  mth: ['mon', 'mth', 'mths', 'month', 'months'],
+  mth: ['mo', 'mon', 'mth', 'mths', 'month', 'months'],
   y: ['y', 'yr', 'yrs', 'year', 'years']
 }