diff --git a/lib/bcoin/bip70/protobuf.js b/lib/bcoin/bip70/protobuf.js
index 9e9a83e0..a077067a 100644
--- a/lib/bcoin/bip70/protobuf.js
+++ b/lib/bcoin/bip70/protobuf.js
@@ -160,18 +160,20 @@ ProtoWriter.prototype.writeVarint = function writeVarint(num) {
   this.writeBytes(buf);
 };
 
-ProtoWriter.prototype.writeFieldValue = function writeFieldValue(tag, value) {
+ProtoWriter.prototype.writeFieldVarint = function writeFieldVarint(tag, value) {
   var header = (tag << 3) | wireType.VARINT;
   this.writeVarint(header);
   this.writeVarint(value);
 };
 
 ProtoWriter.prototype.writeFieldU64 = function writeFieldU64(tag, value) {
-  this.writeFieldValue(tag, value);
+  assert(utils.isSafeInteger(value));
+  this.writeFieldVarint(tag, value);
 };
 
 ProtoWriter.prototype.writeFieldU32 = function writeFieldU32(tag, value) {
-  this.writeFieldValue(tag, value);
+  assert(value <= 0xffffffff);
+  this.writeFieldVarint(tag, value);
 };
 
 ProtoWriter.prototype.writeFieldBytes = function writeFieldBytes(tag, data) {
@@ -199,7 +201,21 @@ exports.readVarint = function readVarint(data, off) {
     }
     ch = data[off++];
     assert(size + 1 < 6, 'Number exceeds 2^53-1.');
-    num += (ch & 0x7f) * Math.pow(2, 7 * size);
+    // Optimization for javascript insanity.
+    switch (size) {
+      case 0:
+      case 1:
+      case 2:
+      case 3:
+        num += (ch & 0x7f) << (7 * size);
+        break;
+      case 4:
+        num += (ch & 0x7f) * (1 << (7 * size));
+        break;
+      default:
+        num += (ch & 0x7f) * Math.pow(2, 7 * size);
+        break;
+    }
     size++;
   }