diff --git a/httplib.h b/httplib.h
index a3d44c9a2c..1fa0943324 100644
--- a/httplib.h
+++ b/httplib.h
@@ -2957,7 +2957,18 @@ inline size_t get_header_value_u64(const Headers &headers,
   std::advance(it, static_cast<ssize_t>(id));
   if (it != rng.second) {
     if (is_numeric(it->second)) {
-      return static_cast<size_t>(std::strtoull(it->second.data(), nullptr, 10));
+      errno = 0;
+      auto val = std::strtoull(it->second.data(), nullptr, 10);
+      auto result = static_cast<size_t>(val);
+      // strtoull saturates to ULLONG_MAX on overflow, and the size_t cast
+      // truncates a value that doesn't fit (a Content-Length above 2^32 wraps
+      // to a small length on 32-bit builds). Either way the framing length
+      // would be wrong, so flag it rather than return a bogus size.
+      if (errno == ERANGE || static_cast<unsigned long long>(result) != val) {
+        is_invalid_value = true;
+        return (std::numeric_limits<size_t>::max)();
+      }
+      return result;
     } else {
       is_invalid_value = true;
     }
diff --git a/test/test.cc b/test/test.cc
index 1410438f63..a0cf231029 100644
--- a/test/test.cc
+++ b/test/test.cc
@@ -1258,6 +1258,26 @@ TEST(GetHeaderValueTest, RegularInvalidValueInt) {
   EXPECT_TRUE(is_invalid_value);
 }
 
+TEST(GetHeaderValueTest, OutOfRangeValueInt) {
+  // An all-digit value that overflows size_t must be reported as invalid, not
+  // silently saturated/truncated: strtoull would otherwise return ULLONG_MAX
+  // (or, on 32-bit builds, the cast would wrap a large length to a small one),
+  // leaving the framing length wrong while is_invalid_value stayed false.
+  Headers headers = {{"Content-Length", "99999999999999999999999999"}};
+  auto is_invalid_value = false;
+  detail::get_header_value_u64(headers, "Content-Length", 0, 0,
+                               is_invalid_value);
+  EXPECT_TRUE(is_invalid_value);
+
+  // A well-formed length is unaffected.
+  Headers ok = {{"Content-Length", "1234"}};
+  is_invalid_value = false;
+  auto val = detail::get_header_value_u64(ok, "Content-Length", 0, 0,
+                                          is_invalid_value);
+  EXPECT_EQ(1234ull, val);
+  EXPECT_FALSE(is_invalid_value);
+}
+
 TEST(GetHeaderValueTest, Range) {
   {
     Headers headers = {make_range_header({{1, -1}})};
@@ -15873,13 +15893,14 @@ TEST(OpenStreamMalformedContentLength, OutOfRange) {
   auto port = port_future.get();
   ASSERT_GT(port, 0);
 
-  // Before the fix, std::stoull would throw std::out_of_range here and
-  // crash the process. After the fix, strtoull silently clamps to
-  // ULLONG_MAX so the stream opens without crashing. The important thing
-  // is that the process does NOT terminate.
+  // Historically std::stoull would throw std::out_of_range here and crash
+  // the process, then strtoull silently clamped to ULLONG_MAX and the
+  // stream opened with a bogus framing length. Now the out-of-range value
+  // is flagged invalid, so the stream fails to open, matching the
+  // not-a-number case above. The process still must NOT terminate.
   Client cli("127.0.0.1", port);
   auto handle = cli.open_stream("GET", "/");
-  EXPECT_TRUE(handle.is_valid());
+  EXPECT_FALSE(handle.is_valid());
 
   server_thread.join();
 }