diff --git a/include/JSystem/JKernel/JKRExpHeap.h b/include/JSystem/JKernel/JKRExpHeap.h
index 1535ba757b..a31b2c5e84 100644
--- a/include/JSystem/JKernel/JKRExpHeap.h
+++ b/include/JSystem/JKernel/JKRExpHeap.h
@@ -35,7 +35,7 @@ public:
         CMemBlock* getNextBlock() const { return mNext; }
         u32 getSize() const { return size; }
         u8 getGroupId() const { return mGroupId; }
-        static CMemBlock* getBlock(void* data) { return (CMemBlock*)((uintptr_t)data + -0x10); }
+        static CMemBlock* getBlock(void* data) { return (CMemBlock*)((uintptr_t)data + -sizeof(CMemBlock)); }
 
     private:
         /* 0x0 */ u16 mMagic;
@@ -44,9 +44,17 @@ public:
         /* 0x4 */ u32 size;
         /* 0x8 */ CMemBlock* mPrev;
         /* 0xC */ CMemBlock* mNext;
+#if BIT_64
+        // Ensure padded to 0x20 bytes on 64-bit
+        void* _pad;
+#endif
     };  // Size: 0x10
     friend class CMemBlock;
 
+#if TARGET_PC
+    static_assert(sizeof(CMemBlock) == MEM_BLOCK_SIZE);
+#endif
+
 protected:
     JKRExpHeap(void* data, u32 size, JKRHeap* parent, bool errorFlag);
     virtual ~JKRExpHeap();
diff --git a/include/JSystem/JKernel/JKRHeap.h b/include/JSystem/JKernel/JKRHeap.h
index c23b72180f..67fb6b8e6d 100644
--- a/include/JSystem/JKernel/JKRHeap.h
+++ b/include/JSystem/JKernel/JKRHeap.h
@@ -16,6 +16,12 @@ extern u8 JKRValue_DEBUGFILL_DELETE;
 extern s32 fillcheck_dispcount;
 extern bool data_8074A8D0_debug;
 
+#if BIT_64
+#define MEM_BLOCK_SIZE 0x20
+#else
+#define MEM_BLOCK_SIZE 0x10
+#endif
+
 /**
  * @ingroup jsystem-jkernel
  * 
diff --git a/src/JSystem/JKernel/JKRHeap.cpp b/src/JSystem/JKernel/JKRHeap.cpp
index 8960b58c7a..1ad3eedceb 100644
--- a/src/JSystem/JKernel/JKRHeap.cpp
+++ b/src/JSystem/JKernel/JKRHeap.cpp
@@ -313,8 +313,8 @@ u8 JKRHeap::getCurrentGroupId() {
 }
 
 u32 JKRHeap::getMaxAllocatableSize(int alignment) {
-    u32 maxFreeBlock = (uintptr_t)getMaxFreeBlock();
-    u32 ptrOffset = (alignment - 1) & alignment - (maxFreeBlock & 0xf);
+    uintptr_t maxFreeBlock = (uintptr_t)getMaxFreeBlock();
+    uintptr_t ptrOffset = (alignment - 1) & alignment - (maxFreeBlock & (MEM_BLOCK_SIZE - 1));
     return ~(alignment - 1) & (getFreeSize() - ptrOffset);
 }