diff --git a/c_tools/mock/generateMock.cpp b/c_tools/mock/generateMock.cpp
index df72e76..fb2ce29 100644
--- a/c_tools/mock/generateMock.cpp
+++ b/c_tools/mock/generateMock.cpp
@@ -349,6 +349,8 @@ void buildBox(SimuData *simu, long num_targets, long loaded,
   for (uint32_t i = 0; i < num_targets; i++, loaded++)
     {
       long pid = particle_id[loaded];
+      assert(pid < simu->NumPart);
+      assert(loaded < boxed->NumPart);
       
       for (int j = 0; j < 3; j++)
 	{
@@ -513,7 +515,7 @@ void makeBoxFromParameter(SimuData *simu, SimuData* &boxed, generateMock_info& a
            {
              particle_id[pid_write] = particle_id[pid_read];
              uniqueID[pid_write] = uniqueID[pid_read];
-             expansion_fac[pid_write] = uniqueID[pid_read];
+             expansion_fac[pid_write] = expansion_fac[pid_read];
              pid_write++; 
            }
           pid_read++;
diff --git a/c_tools/mock/generateMock.ggo b/c_tools/mock/generateMock.ggo
index 3aa591a..fe441e6 100644
--- a/c_tools/mock/generateMock.ggo
+++ b/c_tools/mock/generateMock.ggo
@@ -31,7 +31,7 @@ option "peculiarVelocities" - "Added peculiar velocities distortion" flag off
 
 option "cosmo"	- "Apply cosmological redshift" flag off
 
-option "subsample" - "Subsample the input simulation by the specified amount" double optional
+option "subsample" - "Subsample the input simulation by the specified amount" double optional default="1.0"
 
 option "inputParameter" - "Input geometry (optional, warning!)" string optional
 option "gadgetUnit" - "Unit of length in gadget file in Mpc/h" double optional default="0.001"