diff --git a/lib/bcoin/writer.js b/lib/bcoin/writer.js
index 55260f69..bec886bd 100644
--- a/lib/bcoin/writer.js
+++ b/lib/bcoin/writer.js
@@ -336,7 +336,11 @@ BufferWriter.prototype.writeDoubleBE = function writeDoubleBE(value) {
  */
 
 BufferWriter.prototype.writeVarint = function writeVarint(value) {
-  assert(value >= 0);
+  if (typeof value === 'number')
+    assert(value >= 0);
+  else
+    assert(!value.isNeg());
+
   this.written += utils.sizeVarint(value);
   this.data.push([VARINT, value]);
 };