Index: trunk/ppSim/src/ppSimUtils.c
===================================================================
--- trunk/ppSim/src/ppSimUtils.c	(revision 18011)
+++ trunk/ppSim/src/ppSimUtils.c	(revision 19850)
@@ -228,4 +228,5 @@
 
 // Get a value from the command-line arguments and add it to recipe options
+// if it is not specified, do not override the existing recipe value
 char *ppSimArgToRecipeStr(bool *status,
 			  psMetadata *options,    // Target to which to add value
@@ -241,4 +242,5 @@
 	*status = myStatus;
     }
+    if (!value) return NULL;
     psMetadataAddStr(options, PS_LIST_TAIL, recipeName, PS_META_REPLACE, NULL, value);
     return value;