diff --git a/mrcfile/mrcmemmap.py b/mrcfile/mrcmemmap.py
index 53ad678..cda10b9 100644
--- a/mrcfile/mrcmemmap.py
+++ b/mrcfile/mrcmemmap.py
@@ -114,7 +114,7 @@ def _read_data(self):
     def _open_memmap(self, dtype, shape):
         """Open a new memmap array pointing at the file's data block."""
         acc_mode = 'r' if self._read_only else 'r+'
-        header_nbytes = self.header.nbytes + self.header.nsymbt
+        header_nbytes = int(self.header.nbytes + self.header.nsymbt)
         
         self._iostream.flush()
         try: