diff --git a/js/binary/decoder_test.js b/js/binary/decoder_test.js
index 9900e65bc5ca..8fce993f5d81 100644
--- a/js/binary/decoder_test.js
+++ b/js/binary/decoder_test.js
@@ -315,6 +315,9 @@ describe('binaryDecoderTest', function() {
         // 64-bit extremes, not in dev guide.
         {original: '9223372036854775807', zigzag: '18446744073709551614'},
         {original: '-9223372036854775808', zigzag: '18446744073709551615'},
+        // None of the above catch: bitsLow < 0 && bitsHigh > 0 && bitsHigh < 0x1FFFFF.
+        // The following used to be broken.
+        {original: '72000000000', zigzag: '144000000000'},
       ];
       var encoder = new jspb.BinaryEncoder();
       testCases.forEach(function(c) {
diff --git a/js/binary/utils.js b/js/binary/utils.js
index 6cd15e36445d..4d743310cf55 100644
--- a/js/binary/utils.js
+++ b/js/binary/utils.js
@@ -511,7 +511,7 @@ jspb.utils.joinUnsignedDecimalString = function(bitsLow, bitsHigh) {
   // Skip the expensive conversion if the number is small enough to use the
   // built-in conversions.
   if (bitsHigh <= 0x1FFFFF) {
-    return '' + (jspb.BinaryConstants.TWO_TO_32 * bitsHigh + bitsLow);
+    return '' + jspb.utils.joinUint64(bitsLow, bitsHigh);
   }
 
   // What this code is doing is essentially converting the input number from