diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index 43deb497..0febc002 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -371,13 +371,16 @@ class bigint {
   template <typename UInt, FMT_ENABLE_IF(std::is_same<UInt, uint64_t>::value ||
                                          std::is_same<UInt, uint128_t>::value)>
   FMT_CONSTEXPR20 void multiply(UInt value) {
-    const UInt lower = static_cast<bigit>(value);
-    const UInt upper = value >> bigit_bits;
+    using half_uint =
+        conditional_t<std::is_same<UInt, uint128_t>::value, uint64_t, uint32_t>;
+    const int shift = num_bits<half_uint>() - bigit_bits;
+    const UInt lower = static_cast<half_uint>(value);
+    const UInt upper = value >> num_bits<half_uint>();
     UInt carry = 0;
     for (size_t i = 0, n = bigits_.size(); i < n; ++i) {
       UInt result = lower * bigits_[i] + static_cast<bigit>(carry);
-      carry =
-          upper * bigits_[i] + (result >> bigit_bits) + (carry >> bigit_bits);
+      carry = (upper * bigits_[i] << shift) + (result >> bigit_bits) +
+              (carry >> bigit_bits);
       bigits_[i] = static_cast<bigit>(result);
     }
     while (carry != 0) {
diff --git a/include/fmt/format.h b/include/fmt/format.h
index ac72ad2d..63eed8bb 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -362,9 +362,11 @@ class uint128_fallback {
     result += rhs;
     return result;
   }
-  friend auto operator*(const uint128_fallback&, uint32_t) -> uint128_fallback {
-    FMT_ASSERT(false, "");
-    return {};
+  friend auto operator*(const uint128_fallback& lhs, uint32_t rhs)
+      -> uint128_fallback {
+    FMT_ASSERT(lhs.hi_ == 0, "");
+    uint64_t hi = (lhs.lo_ >> 32) * rhs;
+    return {hi >> 32, (hi << 32) + (lhs.lo_ & ~uint32_t()) * rhs};
   }
   friend auto operator-(const uint128_fallback& lhs, uint64_t rhs)
       -> uint128_fallback {
diff --git a/test/format-impl-test.cc b/test/format-impl-test.cc
index 0276391b..1ce392d0 100644
--- a/test/format-impl-test.cc
+++ b/test/format-impl-test.cc
@@ -101,9 +101,9 @@ TEST(bigint_test, multiply) {
 
   const auto max128 = (fmt::detail::uint128_t(max64) << 64) | max64;
   bigmax = max128;
-  // bigmax *= max128;
-  // EXPECT_EQ(fmt::to_string(bigmax),
-  //          "fffffffffffffffffffffffffffffffe00000000000000000000000000000001");
+  bigmax *= max128;
+  EXPECT_EQ(fmt::to_string(bigmax),
+            "fffffffffffffffffffffffffffffffe00000000000000000000000000000001");
 }
 
 TEST(bigint_test, square) {