diff --git a/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocConstants.hpp b/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocConstants.hpp
index f0a720306b6..a046f46f2b9 100644
--- a/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocConstants.hpp
+++ b/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocConstants.hpp
@@ -24,7 +24,8 @@ inline constexpr const size_t MEDIUM_PAGE_SIZE = (256 * KiB);
 inline constexpr const size_t MEDIUM_PAGE_CELL_COUNT =
         ((MEDIUM_PAGE_SIZE - sizeof(kotlin::alloc::MediumPage)) / sizeof(kotlin::alloc::Cell));
 
-inline constexpr const size_t LARGE_PAGE_SIZE_THRESHOLD = (MEDIUM_PAGE_CELL_COUNT - 1);
+// MEDIUM_PAGE_CELL_COUNT minus one cell for header minus another for the 0-sized dummy block at cells_[0]
+inline constexpr const size_t MEDIUM_PAGE_MAX_BLOCK_SIZE = (MEDIUM_PAGE_CELL_COUNT - 2);
 
 inline constexpr const size_t EXTRA_OBJECT_PAGE_SIZE = 64 * KiB;
 inline constexpr const int EXTRA_OBJECT_COUNT =
diff --git a/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocator.cpp b/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocator.cpp
index c18be2fb9a2..49e9a7e971f 100644
--- a/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocator.cpp
+++ b/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocator.cpp
@@ -12,6 +12,7 @@
 #include <new>
 
 #include "ConcurrentMarkAndSweep.hpp"
+#include "CustomAllocConstants.hpp"
 #include "CustomLogging.hpp"
 #include "ExtraObjectData.hpp"
 #include "ExtraObjectPage.hpp"
@@ -126,7 +127,7 @@ uint8_t* CustomAllocator::Allocate(uint64_t size) noexcept {
     uint8_t* ptr;
     if (cellCount <= SMALL_PAGE_MAX_BLOCK_SIZE) {
         ptr = AllocateInSmallPage(cellCount);
-    } else if (cellCount > LARGE_PAGE_SIZE_THRESHOLD) {
+    } else if (cellCount > MEDIUM_PAGE_MAX_BLOCK_SIZE) {
         ptr = AllocateInLargePage(cellCount);
     } else {
         ptr = AllocateInMediumPage(cellCount);
diff --git a/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocatorTest.cpp b/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocatorTest.cpp
index f93c6fca8c0..48378da66c2 100644
--- a/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocatorTest.cpp
+++ b/kotlin-native/runtime/src/custom_alloc/cpp/CustomAllocatorTest.cpp
@@ -6,6 +6,7 @@
 #include <cstdint>
 #include <random>
 
+#include "CustomAllocConstants.hpp"
 #include "CustomAllocator.hpp"
 #include "GCScheduler.hpp"
 #include "Memory.h"
@@ -55,6 +56,32 @@ TEST(CustomAllocTest, SmallAllocSameSmallPage) {
     }
 }
 
+TEST(CustomAllocTest, SmallPageThreshold) {
+    Heap heap;
+    kotlin::gc::GCSchedulerConfig config;
+    kotlin::gc::GCSchedulerThreadData schedulerData(config, [](auto&) {});
+    CustomAllocator ca(heap, schedulerData);
+    const int FROM = SMALL_PAGE_MAX_BLOCK_SIZE - 10;
+    const int TO = SMALL_PAGE_MAX_BLOCK_SIZE + 10;
+    for (int blocks = FROM; blocks <= TO; ++blocks) {
+        TypeInfo fakeType = {.instanceSize_ = 8 * blocks, .flags_ = 0};
+        ca.CreateObject(&fakeType);
+    }
+}
+
+TEST(CustomAllocTest, MediumPageThreshold) {
+    Heap heap;
+    kotlin::gc::GCSchedulerConfig config;
+    kotlin::gc::GCSchedulerThreadData schedulerData(config, [](auto&) {});
+    CustomAllocator ca(heap, schedulerData);
+    const int FROM = MEDIUM_PAGE_MAX_BLOCK_SIZE - 10;
+    const int TO = MEDIUM_PAGE_MAX_BLOCK_SIZE + 10;
+    for (int blocks = FROM; blocks <= TO; ++blocks) {
+        TypeInfo fakeType = {.instanceSize_ = 8 * blocks, .flags_ = 0};
+        ca.CreateObject(&fakeType);
+    }
+}
+
 TEST(CustomAllocTest, TwoAllocatorsDifferentPages) {
     for (int blocks = MIN_BLOCK_SIZE; blocks < 2000; ++blocks) {
         Heap heap;
diff --git a/kotlin-native/runtime/src/custom_alloc/cpp/LargePage.cpp b/kotlin-native/runtime/src/custom_alloc/cpp/LargePage.cpp
index 6d2e8f5fdff..0b2943b81ff 100644
--- a/kotlin-native/runtime/src/custom_alloc/cpp/LargePage.cpp
+++ b/kotlin-native/runtime/src/custom_alloc/cpp/LargePage.cpp
@@ -16,7 +16,7 @@ namespace kotlin::alloc {
 
 LargePage* LargePage::Create(uint64_t cellCount) noexcept {
     CustomAllocInfo("LargePage::Create(%" PRIu64 ")", cellCount);
-    RuntimeAssert(cellCount > LARGE_PAGE_SIZE_THRESHOLD, "blockSize too small for large page");
+    RuntimeAssert(cellCount > MEDIUM_PAGE_MAX_BLOCK_SIZE, "blockSize too small for large page");
     uint64_t size = sizeof(LargePage) + cellCount * sizeof(uint64_t);
     return new (SafeAlloc(size)) LargePage();
 }