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