diff --git a/sw/simulator/nps/nps_ivy.c b/sw/simulator/nps/nps_ivy.c
index 1e24b303b1..fd5a43a6db 100644
--- a/sw/simulator/nps/nps_ivy.c
+++ b/sw/simulator/nps/nps_ivy.c
@@ -103,7 +103,7 @@ void nps_ivy_send_WORLD_ENV_REQ(void)
 
   int pid = (int)getpid();
   // Bind to the reply
-  ivyPtr = IvyBindMsg(on_WORLD_ENV, NULL, "^%d-%d (\\S*) WORLD_ENV (\\S*) (\\S*) (\\S*) (\\S*) (\\S*) (\\S*)", pid, seq);
+  ivyPtr = IvyBindMsg(on_WORLD_ENV, NULL, "^%d_%d (\\S*) WORLD_ENV (\\S*) (\\S*) (\\S*) (\\S*) (\\S*) (\\S*)", pid, seq);
   // Send actual request
   IvySendMsg("nps %d-%d WORLD_ENV_REQ %f %f %f %f %f %f",
       pid, seq,