diff --git a/wolfcrypt/src/tfm.c b/wolfcrypt/src/tfm.c
index 81372ab8c..be71639f6 100644
--- a/wolfcrypt/src/tfm.c
+++ b/wolfcrypt/src/tfm.c
@@ -2177,13 +2177,20 @@ int mp_div_2d(fp_int* a, int b, fp_int* c, fp_int* d)
 }
 
 #ifdef ALT_ECC_SIZE
-void fp_copy(fp_int *a, fp_int* b)
+void fp_copy(fp_int *a, fp_int *b)
 {
     if (a != b && b->size >= a->used) {
+        int x, oldused;
+        oldused = b->used;
         b->used = a->used;
         b->sign = a->sign;
 
         XMEMCPY(b->dp, a->dp, a->used * sizeof(fp_digit));
+
+        /* zero any excess digits on the destination that we didn't write to */
+        for (x = b->used; x < oldused; x++) {
+            b->dp[x] = 0;
+        }
     }
 }