diff --git a/lib/mining/cpuminer.js b/lib/mining/cpuminer.js
index 07e9aedb..a869f0d6 100644
--- a/lib/mining/cpuminer.js
+++ b/lib/mining/cpuminer.js
@@ -98,6 +98,7 @@ CPUMiner.prototype._close = co(function* close() {
  */
 
 CPUMiner.prototype.start = function start() {
+  assert(!this.running, 'Miner is already running.');
   this._start().catch(util.nop);
 };
 
@@ -206,7 +207,7 @@ CPUMiner.prototype.stop = co(function* stop() {
  * @returns {Promise}
  */
 
-CPUMiner.prototype.stop = co(function* stop() {
+CPUMiner.prototype._stop = co(function* _stop() {
   if (!this.running)
     return;