diff --git a/invesalius/data/slice_.py b/invesalius/data/slice_.py
index 805259d..6a1fa1c 100644
--- a/invesalius/data/slice_.py
+++ b/invesalius/data/slice_.py
@@ -244,6 +244,7 @@ class Slice(object):
     def CloseProject(self):
         self.imagedata = None
         f = self._matrix.filename
+        self._matrix._mmap.close()
         self._matrix  = None
         os.remove(f)
         self.current_mask = None
--
libgit2 0.21.2