diff --git a/api/python/openmatrix/File.py b/api/python/openmatrix/File.py
index 3682463..512af64 100644
--- a/api/python/openmatrix/File.py
+++ b/api/python/openmatrix/File.py
@@ -169,7 +169,7 @@ def createMapping(self, title, entries, overwrite=False):
             self.createGroup(self.root, 'lookup')
 
         # Write the mapping!
-        mymap = self.createArray(self.root.lookup, title, atom=tables.UInt16Atom(),
+        mymap = self.createArray(self.root.lookup, title, atom=tables.UInt32Atom(),
                                  shape=(len(entries),) )
         mymap[:] = entries