diff --git a/include/mbedtls/psa_util.h b/include/mbedtls/psa_util.h
index 8868bc1..a5f09a4 100644
--- a/include/mbedtls/psa_util.h
+++ b/include/mbedtls/psa_util.h
@@ -188,7 +188,8 @@
  *
  * \param       bits        Size of each coordinate in bits.
  * \param       raw         Buffer that contains the signature in raw format.
- * \param       raw_len     Length of \p raw in bytes.
+ * \param       raw_len     Length of \p raw in bytes. This must be
+ *                          PSA_BITS_TO_BYTES(bits) bytes.
  * \param[out]  der         Buffer that will be filled with the converted DER
  *                          output. It can overlap with raw buffer.
  * \param       der_size    Size of \p der in bytes. Given \p bits parameter: