diff --git a/libc/malloc_debug/malloc_debug.cpp b/libc/malloc_debug/malloc_debug.cpp
index b20d634..568192d 100644
--- a/libc/malloc_debug/malloc_debug.cpp
+++ b/libc/malloc_debug/malloc_debug.cpp
@@ -538,13 +538,19 @@
     return g_dispatch->calloc(nmemb, bytes);
   }
 
-  size_t size = nmemb * bytes;
+  size_t size;
+  if (__builtin_mul_overflow(nmemb, bytes, &size)) {
+    // Overflow
+    errno = ENOMEM;
+    return nullptr;
+  }
+
   if (size == 0) {
     size = 1;
   }
 
-  size_t real_size = size + g_debug->extra_bytes();
-  if (real_size < bytes || real_size < nmemb) {
+  size_t real_size;
+  if (__builtin_add_overflow(size, g_debug->extra_bytes(), &real_size)) {
     // Overflow.
     errno = ENOMEM;
     return nullptr;
diff --git a/libc/malloc_debug/tests/malloc_debug_unit_tests.cpp b/libc/malloc_debug/tests/malloc_debug_unit_tests.cpp
index 28729e8..2503981 100644
--- a/libc/malloc_debug/tests/malloc_debug_unit_tests.cpp
+++ b/libc/malloc_debug/tests/malloc_debug_unit_tests.cpp
@@ -1290,6 +1290,12 @@
   ASSERT_TRUE(pointer == nullptr);
   ASSERT_EQ(ENOMEM, errno);
 
+  const size_t size_t_bits = sizeof(size_t) * 8;
+  const size_t sqrt_size_t = 1ULL << (size_t_bits/2);
+  pointer = debug_calloc(sqrt_size_t + 1, sqrt_size_t);
+  ASSERT_TRUE(pointer == nullptr);
+  ASSERT_EQ(ENOMEM, errno);
+
   pointer = debug_realloc(nullptr, SIZE_MAX);
   ASSERT_TRUE(pointer == nullptr);
   ASSERT_EQ(ENOMEM, errno);