diff --git a/lib/bcoin/bip152.js b/lib/bcoin/bip152.js
index 5d462840..5bb2ab33 100644
--- a/lib/bcoin/bip152.js
+++ b/lib/bcoin/bip152.js
@@ -249,7 +249,7 @@ CompactBlock.prototype.initKey = function initKey() {
 };
 
 CompactBlock.prototype.init = function init() {
-  var i, last, ptx, offset;
+  var i, last, ptx, offset, id;
 
   if (this.totalTX === 0)
     throw new Error('Empty vectors.');
@@ -278,15 +278,19 @@ CompactBlock.prototype.init = function init() {
   for (i = 0; i < this.ids.length; i++) {
     while (this.available[i + offset])
       offset++;
-    this.idMap[this.ids[i]] = i + offset;
+
+    id = this.ids[i];
+
+    // Fails on siphash collision
+    assert(!this.idMap[id], 'Siphash collision.');
+
+    this.idMap[id] = i + offset;
+
     // We're supposed to fail here if there's
     // more than 12 hash collisions, but we
     // don't have lowlevel access to our hash
     // table. Hopefully we don't get hashdos'd.
   }
-
-  // Fails on siphash collision
-  assert(this.ids.length === Object.keys(this.idMap).length);
 };
 
 CompactBlock.prototype.toBlock = function toBlock() {