diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index ee06b2f..8beb2af 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -343,6 +343,10 @@
  *
  * \note            For RSA, md_alg may be MBEDTLS_MD_NONE if hash_len != 0.
  *                  For ECDSA, md_alg may never be MBEDTLS_MD_NONE.
+ *
+ * \note            In order to ensure enough space for the signature, the
+ *                  \p sig buffer size must be of at least
+ *                  `max(MBEDTLS_ECDSA_MAX_LEN, MBEDTLS_MPI_MAX_SIZE)` bytes.
  */
 int mbedtls_pk_sign( mbedtls_pk_context *ctx, mbedtls_md_type_t md_alg,
              const unsigned char *hash, size_t hash_len,