diff --git a/library/cipher.c b/library/cipher.c
index f0a770aba..bfb229e2f 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -241,13 +241,13 @@ int cipher_update( cipher_context_t *ctx, const unsigned char *input, size_t ile
 {
     int ret;
 
-    *olen = 0;
-
     if( NULL == ctx || NULL == ctx->cipher_info || NULL == olen )
     {
         return POLARSSL_ERR_CIPHER_BAD_INPUT_DATA;
     }
 
+    *olen = 0;
+
     if( ctx->cipher_info->mode == POLARSSL_MODE_ECB )
     {
         if( ilen != cipher_get_block_size( ctx ) )