diff --git a/owlps-listener/owlps-listenerd.c b/owlps-listener/owlps-listenerd.c
index 9cf68a6..309d831 100644
--- a/owlps-listener/owlps-listenerd.c
+++ b/owlps-listener/owlps-listenerd.c
@@ -1080,9 +1080,10 @@ void extract_calibration_data(const u_char *packet,
                               owl_captured_request *request)
 {
   request->direction = packet[0] ;
+  assert(sizeof(float) == 4) ;
   memcpy(&request->x_position, &packet[1], sizeof(float)) ;
-  memcpy(&request->y_position, &packet[4], sizeof(float)) ;
-  memcpy(&request->z_position, &packet[8], sizeof(float)) ;
+  memcpy(&request->y_position, &packet[5], sizeof(float)) ;
+  memcpy(&request->z_position, &packet[9], sizeof(float)) ;
 }