diff --git a/include/mbedtls/timing.h b/include/mbedtls/timing.h
index 43ce523..597ef75 100644
--- a/include/mbedtls/timing.h
+++ b/include/mbedtls/timing.h
@@ -42,7 +42,7 @@
  * \brief          timer structure
  */
 struct mbedtls_timing_hr_time {
-    uint64_t opaque[4];
+    unsigned char opaque[32];
 };
 
 /**