diff --git a/tpool/aio_simulated.cc b/tpool/aio_simulated.cc
index 93b2ae134b2..6b6fe71c8ab 100644
--- a/tpool/aio_simulated.cc
+++ b/tpool/aio_simulated.cc
@@ -72,7 +72,7 @@ struct WinIoInit
 static WinIoInit win_io_init;
 
 
-int pread(const native_file_handle &h, void *buf, size_t count,
+SSIZE_T pread(const native_file_handle &h, void *buf, size_t count,
                  unsigned long long offset)
 {
   OVERLAPPED ov{};
@@ -81,6 +81,8 @@ int pread(const native_file_handle &h, void *buf, size_t count,
   ov.Offset= uli.LowPart;
   ov.OffsetHigh= uli.HighPart;
   ov.hEvent= win_get_syncio_event();
+  if (count > 0xFFFFFFFF)
+    count= 0xFFFFFFFF;
 
   if (ReadFile(h, buf, (DWORD) count, 0, &ov) ||
       (GetLastError() == ERROR_IO_PENDING))
@@ -93,7 +95,7 @@ int pread(const native_file_handle &h, void *buf, size_t count,
   return -1;
 }
 
-int pwrite(const native_file_handle &h, void *buf, size_t count,
+SSIZE_T pwrite(const native_file_handle &h, void *buf, size_t count,
                   unsigned long long offset)
 {
   OVERLAPPED ov{};
@@ -102,7 +104,8 @@ int pwrite(const native_file_handle &h, void *buf, size_t count,
   ov.Offset= uli.LowPart;
   ov.OffsetHigh= uli.HighPart;
   ov.hEvent= win_get_syncio_event();
-
+  if (count > 0xFFFFFFFF)
+    count= 0xFFFFFFFF;
   if (WriteFile(h, buf, (DWORD) count, 0, &ov) ||
       (GetLastError() == ERROR_IO_PENDING))
   {
diff --git a/tpool/tpool.h b/tpool/tpool.h
index 3a5658c0f36..ed9411e5de5 100644
--- a/tpool/tpool.h
+++ b/tpool/tpool.h
@@ -252,9 +252,9 @@ create_thread_pool_win(int min_threads= DEFAULT_MIN_POOL_THREADS,
   opened with FILE_FLAG_OVERLAPPED, and bound to completion
   port.
 */
-int pwrite(const native_file_handle &h, void *buf, size_t count,
+SSIZE_T pwrite(const native_file_handle &h, void *buf, size_t count,
            unsigned long long offset);
-int pread(const native_file_handle &h, void *buf, size_t count,
+SSIZE_T pread(const native_file_handle &h, void *buf, size_t count,
           unsigned long long offset);
 HANDLE win_get_syncio_event();
 #endif