diff --git a/src/libcore/os.rs b/src/libcore/os.rs
index 5cc7285a7ff..c2de2a646a0 100644
--- a/src/libcore/os.rs
+++ b/src/libcore/os.rs
@@ -763,7 +763,7 @@ struct OverriddenArgs {
     val: ~[~str]
 }
 
-fn overridden_arg_key(+v: @OverriddenArgs) {}
+fn overridden_arg_key(+_v: @OverriddenArgs) {}
 
 pub fn args() -> ~[~str] {
     unsafe {