diff --git a/double-conversion/fixed-dtoa.cc b/double-conversion/fixed-dtoa.cc
index e06617bf..0f55a0b6 100644
--- a/double-conversion/fixed-dtoa.cc
+++ b/double-conversion/fixed-dtoa.cc
@@ -259,6 +259,7 @@ static void FillFractionals(uint64_t fractionals, int exponent,
       fractionals -= static_cast<uint64_t>(digit) << point;
     }
     // If the first bit after the point is set we have to round up.
+    ASSERT(fractionals == 0 || point - 1 >= 0);
     if ((fractionals != 0) && ((fractionals >> (point - 1)) & 1) == 1) {
       RoundUp(buffer, length, decimal_point);
     }
diff --git a/test/cctest/test-fixed-dtoa.cc b/test/cctest/test-fixed-dtoa.cc
index 5ffac581..e66f3890 100644
--- a/test/cctest/test-fixed-dtoa.cc
+++ b/test/cctest/test-fixed-dtoa.cc
@@ -485,6 +485,10 @@ TEST(FastFixedVariousDoubles) {
                       buffer, &length, &point));
   CHECK_EQ("1000000000000000128", buffer.start());
   CHECK_EQ(19, point);
+
+  CHECK(FastFixedDtoa(2.10861548515811875e+15, 17, buffer, &length, &point));
+  CHECK_EQ("210861548515811875", buffer.start());
+  CHECK_EQ(16, point);
 }