diff --git a/lib/blockchainexplorers/insight.js b/lib/blockchainexplorers/insight.js
index 91fad8c..9eb9d67 100644
--- a/lib/blockchainexplorers/insight.js
+++ b/lib/blockchainexplorers/insight.js
@@ -108,7 +108,7 @@ Insight.prototype.getAddressActivity = function(addresses, cb) {
 Insight.prototype.estimateFee = function(nbBlocks, cb) {
   var url = this.url + '/api/utils/estimatefee';
   if (nbBlocks) {
-    url += '?nbBlocks=' + nbBlocks;
+    url += '?nbBlocks=' + [].concat(nbBlocks).join(',');
   }
 
   var args = {