diff --git a/src/impl/win.cc b/src/impl/win.cc
index 786f4f6..889e06f 100644
--- a/src/impl/win.cc
+++ b/src/impl/win.cc
@@ -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;
   }