diff --git a/src/util.h b/src/util.h
index 2f96e2fc27..5f37098b03 100644
--- a/src/util.h
+++ b/src/util.h
@@ -163,10 +163,14 @@ SECP256K1_GNUC_EXT typedef unsigned __int128 uint128_t;
 /* Zero memory if flag == 1. Constant time. */
 static SECP256K1_INLINE void memczero(void *s, size_t len, int flag) {
     unsigned char *p;
-    unsigned char mask = -(unsigned char)flag;
+    /* Access flag with a volatile-qualified lvalue.
+       This prevents clang from figuring out (after inlining) that flag can
+       take only be 0 or 1, which leads to variable time code. */
+    volatile int *flagp = &flag;
+    unsigned char mask = -(unsigned char) *flagp;
     p = (unsigned char *)s;
     while (len) {
-        *p ^= *p & mask;
+        *p &= ~mask;
         p++;
         len--;
     }