diff --git a/library/psa_crypto.c b/library/psa_crypto.c
index 4b17e55..dc0a27d 100644
--- a/library/psa_crypto.c
+++ b/library/psa_crypto.c
@@ -1253,7 +1253,7 @@
 #if defined(MBEDTLS_MD_C)
             if( PSA_ALG_IS_HMAC( operation->alg ) )
             {
-                unsigned int block_size =
+                size_t block_size =
                     psa_get_hash_block_size( PSA_ALG_HMAC_HASH( operation->alg ) );
 
                 if( block_size == 0 )