diff --git a/sys/src/libmp/port/mpadd.c b/sys/src/libmp/port/mpadd.c
index 9a1ccde66..3672c2c34 100644
--- a/sys/src/libmp/port/mpadd.c
+++ b/sys/src/libmp/port/mpadd.c
@@ -21,10 +21,12 @@ mpmagadd(mpint *b1, mpint *b2, mpint *sum)
 	m = b2->top;
 	if(n == 0){
 		mpassign(mpzero, sum);
+		sum->sign = 1;
 		return;
 	}
 	if(m == 0){
 		mpassign(b1, sum);
+		sum->sign = 1;
 		return;
 	}
 	mpbits(sum, (n+1)*Dbits);