diff --git a/musrSim.cc b/musrSim.cc
index de5fb49..f2f0bb1 100644
--- a/musrSim.cc
+++ b/musrSim.cc
@@ -37,6 +37,15 @@
 
 int main(int argc,char** argv) {
 
+  G4String steeringFileName;
+  if (argc > 1) {
+    steeringFileName = argv[1];
+  }
+  else {
+    G4cerr << "Usage: " << argv[0] << " <steering-file> [idle]" << G4endl;
+    return 1;
+  }
+
   //  XInitThreads();
   G4cout<<"\n\n*************************************************************"<<G4endl;
   G4cout<<" musrSim version 1.0.5 for Geant4.10.3, released on 20 Mar 2017"<<G4endl;
@@ -52,7 +61,6 @@ int main(int argc,char** argv) {
   G4RunManager * runManager = new G4RunManager;
 
   // Create class "myParameters", which is a collection of many different parameters
-  G4String steeringFileName=argv[1];
   musrParameters* myParameters = new musrParameters(steeringFileName);
 
   // Create class "musrErrorMessage"
@@ -149,4 +157,3 @@ int main(int argc,char** argv) {
 }
 
 
-