diff --git a/lib/levelup.js b/lib/levelup.js
index 9c3316ccc61..138247e7e87 100644
--- a/lib/levelup.js
+++ b/lib/levelup.js
@@ -328,7 +328,7 @@ LevelUP.prototype.approximateSize = function(start, end, callback) {
       this.emit('error', err)
     } else if (callback)
       callback(null, size)
-  })
+  }.bind(this))
 }
 
 LevelUP.prototype.readStream = function (options) {