diff --git a/bignum.c b/bignum.c
index 144970bba9..8dd70092cc 100644
--- a/bignum.c
+++ b/bignum.c
@@ -1511,15 +1511,16 @@ bigdivrem_mulsub(BDIGIT *zds, size_t zn, BDIGIT x, const BDIGIT *yds, size_t yn)
     i = 0;
 
     do {
-        BDIGIT_DBL ee;
+        BDIGIT_DBL_SIGNED ee;
         t2 += (BDIGIT_DBL)yds[i] * x;
         ee = num - BIGLO(t2);
-        num = (BDIGIT_DBL)zds[i] + ee;
+        num = (BDIGIT_DBL_SIGNED)zds[i] + ee;
         if (ee) zds[i] = BIGLO(num);
         num = BIGDN(num);
         t2 = BIGDN(t2);
     } while (++i < yn);
-    num += zds[i] - t2; /* borrow from high digit; don't update */
+    num -= (BDIGIT_DBL_SIGNED)t2;
+    num += (BDIGIT_DBL_SIGNED)zds[yn]; /* borrow from high digit; don't update */
     return num;
 }