diff --git a/src/secp256k1/src/ecmult_impl.h b/src/secp256k1/src/ecmult_impl.h
--- a/src/secp256k1/src/ecmult_impl.h
+++ b/src/secp256k1/src/ecmult_impl.h
@@ -395,7 +395,7 @@
  *    than the number of bits in the (absolute value) of the input.
  */
 static int secp256k1_ecmult_wnaf(int *wnaf, int len, const secp256k1_scalar *a, int w) {
-    secp256k1_scalar s = *a;
+    secp256k1_scalar s;
     int last_set_bit = -1;
     int bit = 0;
     int sign = 1;
@@ -408,6 +408,7 @@
 
     memset(wnaf, 0, len * sizeof(wnaf[0]));
 
+    s = *a;
     if (secp256k1_scalar_get_bits(&s, 255, 1)) {
         secp256k1_scalar_negate(&s, &s);
         sign = -1;