diff --git a/pyxlsb/workbook.py b/pyxlsb/workbook.py
index 5879d79..73ec94c 100644
--- a/pyxlsb/workbook.py
+++ b/pyxlsb/workbook.py
@@ -42,7 +42,9 @@ def _parse(self):
       reader = BIFF12Reader(fp=temp, debug=self._debug)
       for item in reader:
         if item[0] == biff12.SHEET:
-          self._sheets.append((item[1].name, rels[item[1].rId]))
+          name = item[1].name
+          if rId := item[1].rId:
+            self._sheets.append((name, rels[rId]))
         elif item[0] == biff12.SHEETS_END:
           break
 
@@ -68,13 +70,13 @@ def get_sheet(self, idx, rels=False):
     target = self._sheets[idx - 1][1].split('/')
 
     temp = TemporaryFile()
-    with self._zf.open('xl/{}/{}'.format(target[0], target[-1]), 'r') as zf:
+    with self._zf.open(f'xl/{target[0]}/{target[-1]}', 'r') as zf:
       temp.write(zf.read())
       temp.seek(0, os.SEEK_SET)
 
     if rels:
       rels_temp = TemporaryFile()
-      with self._zf.open('xl/{}/_rels/{}.rels'.format(target[0], target[-1]), 'r') as zf:
+      with self._zf.open(f'xl/{target[0]}/_rels/{target[-1]}.rels', 'r') as zf:
         rels_temp.write(zf.read())
         rels_temp.seek(0, os.SEEK_SET)
     else: