diff --git a/sys/src/libmp/port/mplogic.c b/sys/src/libmp/port/mplogic.c
index 16de7571b..a3258e45b 100644
--- a/sys/src/libmp/port/mplogic.c
+++ b/sys/src/libmp/port/mplogic.c
@@ -141,7 +141,7 @@ mptrunc(mpint *b, int n, mpint *r)
 	m = n % Dbits;
 	if(b->sign == -1){
 		c = 1;
-		for(i = 0; i <= r->top; i++){
+		for(i = 0; i < r->top; i++){
 			if(i < b->top)
 				r->p[i] = ~(b->p[i] - c);
 			else