diff --git a/src/environment/stella_environment.hpp b/src/environment/stella_environment.hpp
index f2a59a0b2..fc4b7278e 100644
--- a/src/environment/stella_environment.hpp
+++ b/src/environment/stella_environment.hpp
@@ -59,7 +59,7 @@ class StellaEnvironment {
    *  number plus the frame skip.
    */
   reward_t act(Action player_a_action, Action player_b_action,
-               float paddle_a_strength = 1.0, float paddle_b_strength = 1.0);
+               float paddle_a_strength, float paddle_b_strength);
 
   /** Applies the given continuous actions (e.g. updating paddle positions when
    * the paddle is used) and performs one simulation step in Stella. Returns the