diff --git a/src/lstm/functions.h b/src/lstm/functions.h
index 92e7e835d..3b09d3f5a 100644
--- a/src/lstm/functions.h
+++ b/src/lstm/functions.h
@@ -149,7 +149,7 @@ struct HPrime {
   }
 };
 struct UnityFunc {
-  inline double operator()(double x) const { return 1.0; }
+  inline double operator()(double /*x*/) const { return 1.0; }
 };
 struct IdentityFunc {
   inline double operator()(double x) const { return x; }