diff --git a/library/psa_util.c b/library/psa_util.c
index 4e350c0..1bb02e9 100644
--- a/library/psa_util.c
+++ b/library/psa_util.c
@@ -346,7 +346,7 @@
  *
  * \param raw_buf           Buffer containing the raw coordinate to be
  *                          converted.
- * \param raw_len           Length of raw_buf in bytes.
+ * \param raw_len           Length of raw_buf in bytes. This must be > 0.
  * \param der_buf_start     Pointer to the beginning of the buffer which
  *                          will be filled with the DER converted data.
  * \param der_buf_end       End of the buffer used to store the DER output.