diff --git a/include/mbedtls/x509_crt.h b/include/mbedtls/x509_crt.h
index 6227460..fb86d4c 100644
--- a/include/mbedtls/x509_crt.h
+++ b/include/mbedtls/x509_crt.h
@@ -570,7 +570,8 @@
  *
  * \return          0 if successful, or MBEDTLS_ERR_X509_ALLOC_FAILED
  */
-int mbedtls_x509write_crt_set_key_usage( mbedtls_x509write_cert *ctx, unsigned char key_usage );
+int mbedtls_x509write_crt_set_key_usage( mbedtls_x509write_cert *ctx,
+                                         unsigned int key_usage );
 
 /**
  * \brief           Set the Netscape Cert Type flags