diff --git a/hll/include/CouponList-internal.hpp b/hll/include/CouponList-internal.hpp
index c92820e2..5b067a59 100644
--- a/hll/include/CouponList-internal.hpp
+++ b/hll/include/CouponList-internal.hpp
@@ -99,6 +99,13 @@ CouponList<A>* CouponList<A>::newList(const void* bytes, size_t len, const A& al
   const bool emptyFlag = ((data[hll_constants::FLAGS_BYTE] & hll_constants::EMPTY_FLAG_MASK) ? true : false);
 
   const uint32_t couponCount = data[hll_constants::LIST_COUNT_BYTE];
+  // Reject LIST counts at or above the fixed LIST capacity.
+  const uint32_t listCapacity = 1u << hll_constants::LG_INIT_LIST_SIZE;
+  if (couponCount >= listCapacity) {
+    throw std::invalid_argument("Attempt to deserialize invalid CouponList with couponCount >= capacity. Found couponCount: "
+                                + std::to_string(couponCount)
+                                + ", capacity: " + std::to_string(listCapacity));
+  }
   const uint32_t couponsInArray = (compact ? couponCount : (1 << HllUtil<A>::computeLgArrInts(LIST, couponCount, lgK)));
   const size_t expectedLength = hll_constants::LIST_INT_ARR_START + (couponsInArray * sizeof(uint32_t));
   if (len < expectedLength) {
@@ -146,11 +153,19 @@ CouponList<A>* CouponList<A>::newList(std::istream& is, const A& allocator) {
   const bool oooFlag = ((listHeader[hll_constants::FLAGS_BYTE] & hll_constants::OUT_OF_ORDER_FLAG_MASK) ? true : false);
   const bool emptyFlag = ((listHeader[hll_constants::FLAGS_BYTE] & hll_constants::EMPTY_FLAG_MASK) ? true : false);
 
+  const uint32_t couponCount = listHeader[hll_constants::LIST_COUNT_BYTE];
+  // Reject LIST counts at or above the fixed LIST capacity.
+  const uint32_t listCapacity = 1u << hll_constants::LG_INIT_LIST_SIZE;
+  if (couponCount >= listCapacity) {
+    throw std::invalid_argument("Attempt to deserialize invalid CouponList with couponCount >= capacity. Found couponCount: "
+                                + std::to_string(couponCount)
+                                + ", capacity: " + std::to_string(listCapacity));
+  }
+
   ClAlloc cla(allocator);
   CouponList<A>* sketch = new (cla.allocate(1)) CouponList<A>(lgK, tgtHllType, mode, allocator);
   using coupon_list_ptr = std::unique_ptr<CouponList<A>, std::function<void(HllSketchImpl<A>*)>>;
   coupon_list_ptr ptr(sketch, sketch->get_deleter());
-  const uint32_t couponCount = listHeader[hll_constants::LIST_COUNT_BYTE];
   sketch->couponCount_ = couponCount;
   sketch->putOutOfOrderFlag(oooFlag); // should always be false for LIST
 
diff --git a/hll/test/CouponListTest.cpp b/hll/test/CouponListTest.cpp
index 81a2bff2..7ad4f700 100644
--- a/hll/test/CouponListTest.cpp
+++ b/hll/test/CouponListTest.cpp
@@ -24,6 +24,7 @@
 #include <string>
 #include <exception>
 #include <stdexcept>
+#include <vector>
 
 #include "hll.hpp"
 #include "CouponList.hpp"
@@ -180,4 +181,67 @@ TEST_CASE("coupon list: check corrupt stream data", "[coupon_list]") {
   REQUIRE_THROWS_AS(hll_sketch::deserialize(ss), std::invalid_argument);
 }
 
+TEST_CASE("coupon list: rejects malformed coupon count in bytes", "[coupon_list]") {
+  // Reject declared LIST counts at or beyond the fixed LIST capacity.
+  uint8_t lgK = 8;
+  hll_sketch sk1(lgK);
+  sk1.update(1);
+  sk1.update(2);
+  const auto validBytes = sk1.serialize_compact();
+
+  const uint8_t listCapacity = static_cast<uint8_t>(1u << hll_constants::LG_INIT_LIST_SIZE);
+  const uint8_t malformedCount = listCapacity;
+  auto sketchBytes = validBytes;
+  // Keep the input long enough to validate the declared LIST count.
+  const size_t requiredLen = hll_constants::LIST_INT_ARR_START
+                             + malformedCount * sizeof(uint32_t);
+  if (sketchBytes.size() < requiredLen) {
+    sketchBytes.resize(requiredLen, 0);
+  }
+  sketchBytes[hll_constants::LIST_COUNT_BYTE] = malformedCount;
+
+  REQUIRE_THROWS_AS(
+      hll_sketch::deserialize(sketchBytes.data(), sketchBytes.size()),
+      std::invalid_argument);
+  REQUIRE_THROWS_AS(
+      CouponList<std::allocator<uint8_t>>::newList(
+          sketchBytes.data(), sketchBytes.size(), std::allocator<uint8_t>()),
+      std::invalid_argument);
+}
+
+TEST_CASE("coupon list: rejects malformed coupon count in stream", "[coupon_list]") {
+  uint8_t lgK = 8;
+  hll_sketch sk1(lgK);
+  sk1.update(1);
+  sk1.update(2);
+
+  const uint8_t listCapacity = static_cast<uint8_t>(1u << hll_constants::LG_INIT_LIST_SIZE);
+  const uint8_t malformedCount = listCapacity;
+  std::stringstream ss(std::ios::in | std::ios::out | std::ios::binary);
+  sk1.serialize_compact(ss);
+
+  const size_t requiredLen = hll_constants::LIST_INT_ARR_START
+                             + malformedCount * sizeof(uint32_t);
+  // Keep the stream long enough to validate the declared LIST count.
+  ss.seekp(0, std::ios::end);
+  const auto endPos = static_cast<size_t>(ss.tellp());
+  if (endPos < requiredLen) {
+    std::vector<char> pad(requiredLen - endPos, 0);
+    ss.write(pad.data(), pad.size());
+  }
+
+  ss.seekp(hll_constants::LIST_COUNT_BYTE);
+  ss.put(static_cast<char>(malformedCount));
+
+  ss.clear();
+  ss.seekg(0);
+  REQUIRE_THROWS_AS(hll_sketch::deserialize(ss), std::invalid_argument);
+
+  ss.clear();
+  ss.seekg(0);
+  REQUIRE_THROWS_AS(
+      CouponList<std::allocator<uint8_t>>::newList(ss, std::allocator<uint8_t>()),
+      std::invalid_argument);
+}
+
 } /* namespace datasketches */