diff --git a/library/pk.c b/library/pk.c
index ecf002d..05cc213 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -235,12 +235,15 @@
 {
     const mbedtls_md_info_t *md_info;
 
-    if( *hash_len != 0 )
+    if( *hash_len != 0 && md_alg == MBEDTLS_MD_NONE )
         return( 0 );
 
     if( ( md_info = mbedtls_md_info_from_type( md_alg ) ) == NULL )
         return( -1 );
 
+    if ( *hash_len != 0 && *hash_len != mbedtls_md_get_size( md_info ) )
+        return ( -1 );
+
     *hash_len = mbedtls_md_get_size( md_info );
     return( 0 );
 }