diff --git a/dlls/kernel32/heap.c b/dlls/kernel32/heap.c
index dfaf69a5385..610cff08388 100644
--- a/dlls/kernel32/heap.c
+++ b/dlls/kernel32/heap.c
@@ -287,7 +287,7 @@ LPVOID WINAPI HeapReAlloc( HANDLE heap, DWORD flags, LPVOID ptr, SIZE_T size )
     return RtlReAllocateHeap( heap, flags, ptr, size );
 }
 
-SIZE_T WINAPI HeapSize( HANDLE heap, DWORD flags, LPVOID ptr )
+SIZE_T WINAPI HeapSize( HANDLE heap, DWORD flags, LPCVOID ptr )
 {
     return RtlSizeHeap( heap, flags, ptr );
 }
diff --git a/dlls/ntdll/heap.c b/dlls/ntdll/heap.c
index 25aefd9f3ae..571dd23a4e2 100644
--- a/dlls/ntdll/heap.c
+++ b/dlls/ntdll/heap.c
@@ -1511,7 +1511,7 @@ BOOLEAN WINAPI RtlUnlockHeap( HANDLE heap )
  * NOTES
  *  The size may be bigger than what was passed to RtlAllocateHeap().
  */
-SIZE_T WINAPI RtlSizeHeap( HANDLE heap, ULONG flags, PVOID ptr )
+SIZE_T WINAPI RtlSizeHeap( HANDLE heap, ULONG flags, const void *ptr )
 {
     SIZE_T ret;
     HEAP *heapPtr = HEAP_GetPtr( heap );
@@ -1531,7 +1531,7 @@ SIZE_T WINAPI RtlSizeHeap( HANDLE heap, ULONG flags, PVOID ptr )
     }
     else
     {
-        ARENA_INUSE *pArena = (ARENA_INUSE *)ptr - 1;
+        const ARENA_INUSE *pArena = (const ARENA_INUSE *)ptr - 1;
         ret = (pArena->size & ARENA_SIZE_MASK) - pArena->unused_bytes;
     }
     if (!(flags & HEAP_NO_SERIALIZE)) RtlLeaveCriticalSection( &heapPtr->critSection );
diff --git a/include/winbase.h b/include/winbase.h
index da9c547bacd..c45fbf119f3 100644
--- a/include/winbase.h
+++ b/include/winbase.h
@@ -1742,7 +1742,7 @@ BOOL        WINAPI HeapLock(HANDLE);
 LPVOID      WINAPI HeapReAlloc(HANDLE,DWORD,LPVOID,SIZE_T);
 BOOL        WINAPI HeapQueryInformation(HANDLE,HEAP_INFORMATION_CLASS,PVOID,SIZE_T,PSIZE_T);
 BOOL        WINAPI HeapSetInformation(HANDLE,HEAP_INFORMATION_CLASS,PVOID,SIZE_T);
-SIZE_T      WINAPI HeapSize(HANDLE,DWORD,LPVOID);
+SIZE_T      WINAPI HeapSize(HANDLE,DWORD,LPCVOID);
 BOOL        WINAPI HeapUnlock(HANDLE);
 BOOL        WINAPI HeapValidate(HANDLE,DWORD,LPCVOID);
 BOOL        WINAPI HeapWalk(HANDLE,LPPROCESS_HEAP_ENTRY);
diff --git a/include/winternl.h b/include/winternl.h
index 1dc60115576..fa3696f5988 100644
--- a/include/winternl.h
+++ b/include/winternl.h
@@ -2193,7 +2193,7 @@ void      WINAPI RtlSetLastWin32Error(DWORD);
 void      WINAPI RtlSetLastWin32ErrorAndNtStatusFromNtStatus(NTSTATUS);
 NTSTATUS  WINAPI RtlSetSaclSecurityDescriptor(PSECURITY_DESCRIPTOR,BOOLEAN,PACL,BOOLEAN);
 NTSTATUS  WINAPI RtlSetTimeZoneInformation(const RTL_TIME_ZONE_INFORMATION*);
-SIZE_T    WINAPI RtlSizeHeap(HANDLE,ULONG,PVOID);
+SIZE_T    WINAPI RtlSizeHeap(HANDLE,ULONG,const void*);
 NTSTATUS  WINAPI RtlStringFromGUID(REFGUID,PUNICODE_STRING);
 LPDWORD   WINAPI RtlSubAuthoritySid(PSID,DWORD);
 LPBYTE    WINAPI RtlSubAuthorityCountSid(PSID);