diff --git a/include/mbedtls/aria.h b/include/mbedtls/aria.h
index bcbc03da5..b3617505d 100644
--- a/include/mbedtls/aria.h
+++ b/include/mbedtls/aria.h
@@ -63,7 +63,7 @@ extern "C" {
  */
 typedef struct
 {
-    int nr;                 /*!< The number of rounds (12, 14 or 16) */
+    unsigned char nr;           /*!< The number of rounds (12, 14 or 16) */
     /*! The ARIA round keys. */
     uint32_t rk[MBEDTLS_ARIA_MAX_ROUNDS + 1][MBEDTLS_ARIA_BLOCKSIZE / 4];
 }