diff --git a/mach/proto/as/comm6.c b/mach/proto/as/comm6.c
index def1eb5b8..bb1fe4499 100644
--- a/mach/proto/as/comm6.c
+++ b/mach/proto/as/comm6.c
@@ -87,7 +87,7 @@ register item_t *ip;
 	if (store(ip, (valu_t) DOTVAL) == 0)
 		return;
 #ifdef THREE_PASS
-	assert(pass != PASS_2 || oldval - ip->i_valu == DOTGAIN);
+	assert(pass != PASS_2 || (oldval -= ip->i_valu) == DOTGAIN);
 #endif
 }