diff --git a/atrcopy.py b/atrcopy.py
index ec387e1..a4fc134 100755
--- a/atrcopy.py
+++ b/atrcopy.py
@@ -68,7 +68,7 @@ class AtrHeader(object):
         self.max_sectors = self.size_in_bytes / self.sector_size
     
     def sector_is_valid(self, sector):
-        return sector >= 0 and sector < self.max_sectors
+        return sector > 0 and sector <= self.max_sectors
     
     def get_pos(self, sector):
         if not self.sector_is_valid(sector):