diff --git a/bots/check-mirroring b/bots/check-mirroring
index 78a2b66..a0ff373 100755
--- a/bots/check-mirroring
+++ b/bots/check-mirroring
@@ -11,6 +11,8 @@ import subprocess
 import hashlib
 from six.moves import range
 
+if False: from typing import Any, Dict, List, Set, Tuple
+
 parser = optparse.OptionParser()
 parser.add_option('--verbose',
                   dest='verbose',
@@ -91,7 +93,7 @@ if options.sharded:
     for (stream, test) in test_streams:
         if stream == "message":
             continue
-        assert(hashlib.sha1(stream).hexdigest().startswith(test))
+        assert(hashlib.sha1(stream.encode("utf-8")).hexdigest().startswith(test))
 else:
     test_streams = [
         ("message", "p"),
@@ -99,6 +101,8 @@ else:
         ]
 
 def print_status_and_exit(status):
+    # type: (int) -> None
+
     # The output of this script is used by Nagios. Various outputs,
     # e.g. true success and punting due to a SERVNAK, result in a
     # non-alert case, so to give us something unambiguous to check in
@@ -107,15 +111,17 @@ def print_status_and_exit(status):
     sys.exit(status)
 
 def send_zulip(message):
+    # type: (Dict[str, str]) -> None
     result = zulip_client.send_message(message)
     if result["result"] != "success":
         logger.error("Error sending zulip, args were:")
-        logger.error(message)
+        logger.error(message) # type: ignore # https://github.com/python/typeshed/issues/532
         logger.error(result)
         print_status_and_exit(1)
 
 # Returns True if and only if we "Detected server failure" sending the zephyr.
 def send_zephyr(zwrite_args, content):
+    # type: (List[str], str) -> bool
     p = subprocess.Popen(zwrite_args, stdin=subprocess.PIPE,
                          stdout=subprocess.PIPE, stderr=subprocess.PIPE)
     stdout, stderr = p.communicate(input=content.encode("utf-8"))
@@ -167,7 +173,7 @@ for tries in range(10):
             actually_subscribed = True
             break
     except IOError as e:
-        if "SERVNAK received" in e:
+        if "SERVNAK received" in e: # type: ignore # https://github.com/python/mypy/issues/2118
             logger.error("SERVNAK repeatedly received, punting rest of test")
         else:
             logger.exception("Exception subscribing to zephyrs")
@@ -177,9 +183,10 @@ if not actually_subscribed:
     print_status_and_exit(1)
 
 # Prepare keys
-zhkeys = {}
-hzkeys = {}
+zhkeys = {} # type: Dict[str, Tuple[str, str]]
+hzkeys = {} # type: Dict[str, Tuple[str, str]]
 def gen_key(key_dict):
+    # type: (Dict[str, Any]) -> str
     bits = str(random.getrandbits(32))
     while bits in key_dict:
         # Avoid the unlikely event that we get the same bits twice
@@ -187,6 +194,7 @@ def gen_key(key_dict):
     return bits
 
 def gen_keys(key_dict):
+    # type: (Dict[str, Tuple[str, str]]) -> None
     for (stream, test) in test_streams:
         key_dict[gen_key(key_dict)] = (stream, test)
 
@@ -199,6 +207,7 @@ notices = []
 # receive queue with 30+ messages, which might result in messages
 # being dropped.
 def receive_zephyrs():
+    # type: () -> None
     while True:
         try:
             notice = zephyr.receive(block=False)
@@ -281,10 +290,12 @@ logger.info("Finished receiving Zephyr messages!")
 
 all_keys = set(list(zhkeys.keys()) + list(hzkeys.keys()))
 def process_keys(content_list):
+    # type: (List[str]) -> Tuple[Dict[str, int], Set[str], Set[str], bool, bool]
+
     # Start by filtering out any keys that might have come from
     # concurrent check-mirroring processes
     content_keys = [key for key in content_list if key in all_keys]
-    key_counts = {}
+    key_counts = {} # type: Dict[str, int]
     for key in all_keys:
         key_counts[key] = 0
     for key in content_keys: