diff --git a/src/main.c b/src/main.c
index 4f358bfef..14f8af18c 100644
--- a/src/main.c
+++ b/src/main.c
@@ -277,6 +277,7 @@ void clear_sim(void)
 	memset(bmap, 0, sizeof(bmap));
 	memset(emap, 0, sizeof(emap));
 	memset(signs, 0, sizeof(signs));
+	MSIGN = -1;
 	memset(parts, 0, sizeof(particle)*NPART);
 	for (i=0; i<NPART-1; i++)
 		parts[i].life = i+1;