diff --git a/include/psa/crypto_struct.h b/include/psa/crypto_struct.h
index fdf78a8..0ddc7a3 100644
--- a/include/psa/crypto_struct.h
+++ b/include/psa/crypto_struct.h
@@ -210,7 +210,7 @@
      * hence we must store it for the lifetime of the operation.
      * This is different from HKDF, where the key is only used
      * in the extraction phase, but not during expansion. */
-    unsigned char *key;
+    uint8_t *key;
     size_t key_len;
 
     /* `A(i) + seed` in the notation of RFC 5246, Sect. 5 */