diff --git a/src/impl/win.cc b/src/impl/win.cc
index d933fe9..c25bf4f 100644
--- a/src/impl/win.cc
+++ b/src/impl/win.cc
@@ -317,7 +317,7 @@ Serial::SerialImpl::read (uint8_t *buf, size_t size)
     throw PortNotOpenedException ("Serial::read");
   }
   DWORD bytes_read;
-  if (!ReadFile(fd_, buf, size, &bytes_read, NULL)) {
+  if (!ReadFile(fd_, buf, static_cast<DWORD>(size), &bytes_read, NULL)) {
     stringstream ss;
     ss << "Error while reading from the serial port: " << GetLastError();
     THROW (IOException, ss.str().c_str());
@@ -332,7 +332,7 @@ Serial::SerialImpl::write (const uint8_t *data, size_t length)
     throw PortNotOpenedException ("Serial::write");
   }
   DWORD bytes_written;
-  if (!WriteFile(fd_, data, length, &bytes_written, NULL)) {
+  if (!WriteFile(fd_, data, static_cast<DWORD>(length), &bytes_written, NULL)) {
     stringstream ss;
     ss << "Error while writing to the serial port: " << GetLastError();
     THROW (IOException, ss.str().c_str());