diff --git a/lib/bcoin/bip70/protobuf.js b/lib/bcoin/bip70/protobuf.js
index a077067a..29dac687 100644
--- a/lib/bcoin/bip70/protobuf.js
+++ b/lib/bcoin/bip70/protobuf.js
@@ -200,7 +200,6 @@ exports.readVarint = function readVarint(data, off) {
       break;
     }
     ch = data[off++];
-    assert(size + 1 < 6, 'Number exceeds 2^53-1.');
     // Optimization for javascript insanity.
     switch (size) {
       case 0:
@@ -219,6 +218,8 @@ exports.readVarint = function readVarint(data, off) {
     size++;
   }
 
+  assert(utils.isSafeInteger(num), 'Number exceeds 2^53-1.');
+
   return { size: size, value: num };
 };