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 = {
diff --git a/lib/server.js b/lib/server.js
index cd85a5f..7c19bf9 100644
--- a/lib/server.js
+++ b/lib/server.js
@@ -752,23 +752,22 @@ WalletService.prototype.getBalance = function(opts, cb) {
 WalletService.prototype._sampleFeeLevels = function(network, points, cb) {
   var self = this;
 
-  // TODO: cache blockexplorer data
   var bc = self._getBlockchainExplorer(network);
-  async.map(points, function(p, next) {
-    bc.estimateFee(p, function(err, result) {
-      if (err) {
-        log.error('Error estimating fee', err);
-        return next(err);
-      }
-      var feePerKB = _.isObject(result) ? +(result.feePerKB) : -1;
+  bc.estimateFee(points, function(err, result) {
+    if (err) {
+      log.error('Error estimating fee', err);
+      return cb(err);
+    }
+
+    var levels = _.zipObject(_.map(points, function(p) {
+      var feePerKB = _.isObject(result) ? +result[p] : -1;
       if (feePerKB < 0) {
         log.warn('Could not compute fee estimation (nbBlocks=' + p + ')');
       }
-      return next(null, [p, Utils.strip(feePerKB * 1e8)]);
-    });
-  }, function(err, results) {
-    if (err) return cb(err);
-    return cb(null, _.zipObject(results));
+      return [p, Utils.strip(feePerKB * 1e8)];
+    }));
+
+    return cb(null, levels);
   });
 };
 
diff --git a/test/integration/server.js b/test/integration/server.js
index b6be0e9..da43d8b 100644
--- a/test/integration/server.js
+++ b/test/integration/server.js
@@ -196,9 +196,10 @@ helpers.stubHistory = function(txs) {
 
 helpers.stubFeeLevels = function(levels) {
   blockchainExplorer.estimateFee = function(nbBlocks, cb) {
-    return cb(null, {
-      feePerKB: levels[nbBlocks] / 1e8
-    });
+    var result = _.zipObject(_.map(_.pick(levels, nbBlocks), function(fee, n) {
+      return [+n, fee > 0 ? fee / 1e8 : fee];
+    }));
+    return cb(null, result);
   };
 };