diff --git a/src/impl/win.cpp b/src/impl/win.cpp
index f5e00fcc..c29d49d1 100644
--- a/src/impl/win.cpp
+++ b/src/impl/win.cpp
@@ -24,7 +24,7 @@ inline wstring
 _prefix_port_if_needed(const wstring &input)
 {
   static wstring windows_com_port_prefix = L"\\\\.\\";
-  if (input.compare(windows_com_port_prefix) != 0)
+  if (input.compare(0, windows_com_port_prefix.size(), windows_com_port_prefix) != 0)
   {
     return windows_com_port_prefix + input;
   }