diff --git a/include/mbedtls/pk.h b/include/mbedtls/pk.h
index 3a35afb..d70e546 100644
--- a/include/mbedtls/pk.h
+++ b/include/mbedtls/pk.h
@@ -503,7 +503,11 @@
  * \param pub       Context holding a public key.
  * \param prv       Context holding a private (and public) key.
  *
- * \return          0 on success or MBEDTLS_ERR_PK_BAD_INPUT_DATA
+ * \return          \c 0 on success (keys were checked and match each other).
+ * \return          #MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE if the keys could not
+ *                  be checked - in that case they may or may not match.
+ * \return          #MBEDTLS_ERR_PK_BAD_INPUT_DATA if a context is invalid.
+ * \return          Another non-zero value if the keys do not match.
  */
 int mbedtls_pk_check_pair( const mbedtls_pk_context *pub, const mbedtls_pk_context *prv );
 
diff --git a/library/pk.c b/library/pk.c
index cb6e158..b2f6812 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -456,12 +456,14 @@
 int mbedtls_pk_check_pair( const mbedtls_pk_context *pub, const mbedtls_pk_context *prv )
 {
     if( pub == NULL || pub->pk_info == NULL ||
-        prv == NULL || prv->pk_info == NULL ||
-        prv->pk_info->check_pair_func == NULL )
+        prv == NULL || prv->pk_info == NULL )
     {
         return( MBEDTLS_ERR_PK_BAD_INPUT_DATA );
     }
 
+    if( prv->pk_info->check_pair_func == NULL )
+        return( MBEDTLS_ERR_PK_FEATURE_UNAVAILABLE );
+
     if( prv->pk_info->type == MBEDTLS_PK_RSA_ALT )
     {
         if( pub->pk_info->type != MBEDTLS_PK_RSA )