diff --git a/wfdb/io/_header.py b/wfdb/io/_header.py
index a5dc2872..a1feb5f2 100644
--- a/wfdb/io/_header.py
+++ b/wfdb/io/_header.py
@@ -1,5 +1,5 @@
 import datetime
-from typing import Collection, List, Tuple
+from typing import Any, Dict, List, Optional, Sequence, Tuple
 
 import numpy as np
 import pandas as pd
@@ -598,6 +598,10 @@ class MultiHeaderMixin(BaseHeaderMixin):
 
     """
 
+    n_seg: int
+    seg_len: Sequence[int]
+    segments: Optional[Sequence]
+
     def set_defaults(self):
         """
         Set defaults for fields needed to write the header if they have
@@ -920,7 +924,7 @@ def contained_ranges(self, sig_name: str) -> List[Tuple[int, int]]:
 
     def contained_combined_ranges(
         self,
-        sig_names: Collection[str],
+        sig_names: Sequence[str],
     ) -> List[Tuple[int, int]]:
         """
         Given a collection of signal name, return the sample ranges that
@@ -1010,7 +1014,7 @@ def _parse_record_line(record_line: str) -> dict:
 
     """
     # Dictionary for record fields
-    record_fields = {}
+    record_fields: Dict[str, Any] = {}
 
     # Read string fields from record line
     match = rx_record.match(record_line)
diff --git a/wfdb/io/download.py b/wfdb/io/download.py
index ace112f9..d494ad0e 100644
--- a/wfdb/io/download.py
+++ b/wfdb/io/download.py
@@ -23,7 +23,7 @@ class Config(object):
 
     """
 
-    pass
+    db_index_url: str
 
 
 # The configuration database index url. Uses PhysioNet index by default.
diff --git a/wfdb/io/record.py b/wfdb/io/record.py
index 860c908d..94cae73b 100644
--- a/wfdb/io/record.py
+++ b/wfdb/io/record.py
@@ -1661,7 +1661,7 @@ def multi_to_single(self, physical, return_res=64, expanded=False):
 # this library
 ALLOWED_TYPES = dict(
     [
-        [index, _header.FIELD_SPECS.loc[index, "allowed_types"]]
+        (index, _header.FIELD_SPECS.loc[index, "allowed_types"])
         for index in _header.FIELD_SPECS.index
     ]
 )
diff --git a/wfdb/io/util.py b/wfdb/io/util.py
index 12ecde33..1b3f4ad9 100644
--- a/wfdb/io/util.py
+++ b/wfdb/io/util.py
@@ -4,7 +4,7 @@
 import math
 import os
 
-from typing import Sequence, Tuple
+from typing import List, Sequence, Tuple
 
 
 def lines_to_file(file_name: str, write_dir: str, lines: Sequence[str]):
@@ -102,8 +102,9 @@ def upround(x, base):
 
 
 def overlapping_ranges(
-    ranges_1: Tuple[int, int], ranges_2: Tuple[int, int]
-) -> Tuple[int, int]:
+    ranges_1: Sequence[Tuple[int, int]],
+    ranges_2: Sequence[Tuple[int, int]],
+) -> List[Tuple[int, int]]:
     """
     Given two collections of integer ranges, return a list of ranges
     in which both input inputs overlap.