diff --git a/library/common.h b/library/common.h
index fd3ddba..ae6625b 100644
--- a/library/common.h
+++ b/library/common.h
@@ -122,11 +122,13 @@
  */
 inline void mbedtls_xor(unsigned char *r, const unsigned char *a, const unsigned char *b, size_t n)
 {
-    size_t i;
-    for (i = 0; (i + 4) <= n; i += 4) {
+    size_t i = 0;
+#if defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)
+    for (; (i + 4) <= n; i += 4) {
         uint32_t x = mbedtls_get_unaligned_uint32(a + i) ^ mbedtls_get_unaligned_uint32(b + i);
         mbedtls_put_unaligned_uint32(r + i, x);
     }
+#endif
     for (; i < n; i++) {
         r[i] = a[i] ^ b[i];
     }