diff --git a/build.lua b/build.lua
index 4f8c115..ae4096a 100644
--- a/build.lua
+++ b/build.lua
@@ -14,7 +14,7 @@ else
 end
 
 local ext = os_uname.sysname
-if os_uname.sysname == "Windows" then ext = "dll"
+if os_uname.sysname == "Windows_NT" then ext = "dll"
 elseif os_uname.sysname == "Mac" then ext = "dylib"
 else ext = "so"
 end