diff --git a/src/PenningTrap.cpp b/src/PenningTrap.cpp
index d0478ec..20e2db1 100644
--- a/src/PenningTrap.cpp
+++ b/src/PenningTrap.cpp
@@ -238,7 +238,7 @@ sim_arr PenningTrap::simulate(double time, unsigned int steps,
 }
 
 void PenningTrap::write_simulation_to_dir(std::string path, double time,
-                                          int steps, std::string method,
+                                          unsigned int steps, std::string method,
                                           bool particle_interaction)
 {
     if (path.back() != '/') {