diff --git a/library/ecp_invasive.h b/library/ecp_invasive.h
index 16b7b61..aadcdbc 100644
--- a/library/ecp_invasive.h
+++ b/library/ecp_invasive.h
@@ -184,6 +184,8 @@
  * \param[in]       X_limbs The length of \p X in limbs.
  *
  * \return          \c 0 on success.
+ * \return          #MBEDTLS_ERR_ECP_BAD_INPUT_DATA if \p X does not have
+ *                  twice as many limbs as the modulus.
  * \return          #MBEDTLS_ERR_ECP_ALLOC_FAILED if memory allocation failed.
  */
 MBEDTLS_STATIC_TESTABLE
@@ -206,6 +208,8 @@
  * \param[in]       X_limbs The length of \p X in limbs.
  *
  * \return          \c 0 on success.
+ * \return          #MBEDTLS_ERR_ECP_BAD_INPUT_DATA if \p X does not have
+ *                  twice as many limbs as the modulus.
  * \return          #MBEDTLS_ERR_ECP_ALLOC_FAILED if memory allocation failed.
  */
 MBEDTLS_STATIC_TESTABLE
@@ -228,6 +232,8 @@
  * \param[in]       X_limbs The length of \p X in limbs.
  *
  * \return          \c 0 on success.
+ * \return          #MBEDTLS_ERR_ECP_BAD_INPUT_DATA if \p X does not have
+ *                  twice as many limbs as the modulus.
  * \return          #MBEDTLS_ERR_ECP_ALLOC_FAILED if memory allocation failed.
  */
 MBEDTLS_STATIC_TESTABLE