diff --git a/library/cipher.c b/library/cipher.c
index b69d331..7ea25cf 100644
--- a/library/cipher.c
+++ b/library/cipher.c
@@ -315,9 +315,9 @@
          * If there is not enough data for a full block, cache it.
          */
         if( ( ctx->operation == POLARSSL_DECRYPT &&
-                ilen + ctx->unprocessed_len <= cipher_get_block_size( ctx ) ) ||
+                ilen <= cipher_get_block_size( ctx ) - ctx->unprocessed_len ) ||
              ( ctx->operation == POLARSSL_ENCRYPT &&
-                ilen + ctx->unprocessed_len < cipher_get_block_size( ctx ) ) )
+                ilen < cipher_get_block_size( ctx ) - ctx->unprocessed_len ) )
         {
             memcpy( &( ctx->unprocessed_data[ctx->unprocessed_len] ), input,
                     ilen );