diff --git a/library/bignum.c b/library/bignum.c
index 5ec0541..699d2c6 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1280,7 +1280,7 @@
     /* Set the high limbs of X to match A. Don't touch the lower limbs
      * because X might be aliased to B, and we must not overwrite the
      * significant digits of B. */
-    if (A->n > n) {
+    if (A->n > n && A != X) {
         memcpy(X->p + n, A->p + n, (A->n - n) * ciL);
     }
     if (X->n > A->n) {