diff --git a/lib/trinity/sim_file.ex b/lib/trinity/sim_file.ex
index d8b03fb..0673c4d 100644
--- a/lib/trinity/sim_file.ex
+++ b/lib/trinity/sim_file.ex
@@ -156,7 +156,7 @@ defmodule Trinity.SimFile do
     start_block_index = div(loc, @block_size)
     start_pos = rem(loc, @block_size)
 
-    case (loc + bin_size) <= @block_size do
+    case (start_pos + bin_size) <= @block_size do
       true ->
         write_block(file_data, fd, start_block_index, start_pos, bin)