diff --git a/bw_add_sshkeys.py b/bw_add_sshkeys.py
index 6601f15..c3d889b 100755
--- a/bw_add_sshkeys.py
+++ b/bw_add_sshkeys.py
@@ -8,17 +8,18 @@ import json
 import logging
 import os
 import subprocess
+from typing import Any, Callable, Dict, List
 
 from pkg_resources import parse_version
 
 
-def memoize(func):
+def memoize(func: Callable[..., Any]) -> Callable[..., Any]:
     """
     Decorator function to cache the results of another function call
     """
-    cache = dict()
+    cache: Dict[Any, Callable[..., Any]] = {}
 
-    def memoized_func(*args):
+    def memoized_func(*args: Any) -> Any:
         if args in cache:
             return cache[args]
         result = func(*args)
@@ -29,7 +30,7 @@ def memoize(func):
 
 
 @memoize
-def bwcli_version():
+def bwcli_version() -> str:
     """
     Function to return the version of the Bitwarden CLI
     """
@@ -43,7 +44,7 @@ def bwcli_version():
 
 
 @memoize
-def cli_supports(feature):
+def cli_supports(feature: str) -> bool:
     """
     Function to return whether the current Bitwarden CLI supports a particular
     feature
@@ -55,7 +56,7 @@ def cli_supports(feature):
     return False
 
 
-def get_session():
+def get_session() -> str:
     """
     Function to return a valid Bitwarden session
     """
@@ -66,7 +67,7 @@ def get_session():
         return session
 
     # Check if we're already logged in
-    proc_logged = subprocess.run(['bw', 'login', '--check', '--quiet'])
+    proc_logged = subprocess.run(['bw', 'login', '--check', '--quiet'], check=True)
 
     if proc_logged.returncode:
         logging.debug('Not logged into Bitwarden')
@@ -89,7 +90,7 @@ def get_session():
     return session
 
 
-def get_folders(session, foldername):
+def get_folders(session: str, foldername: str) -> str:
     """
     Function to return the ID  of the folder that matches the provided name
     """
@@ -106,17 +107,17 @@ def get_folders(session, foldername):
 
     if not folders:
         logging.error('"%s" folder not found', foldername)
-        return None
+        return ''
 
     # Do we have any folders
     if len(folders) != 1:
         logging.error('%d folders with the name "%s" found', len(folders), foldername)
-        return None
+        return ''
 
-    return folders[0]['id']
+    return str(folders[0]['id'])
 
 
-def folder_items(session, folder_id):
+def folder_items(session: str, folder_id: str) -> List[Dict[str, Any]]:
     """
     Function to return items from a folder
     """
@@ -128,10 +129,13 @@ def folder_items(session, folder_id):
         universal_newlines=True,
         check=True,
     )
-    return json.loads(proc_items.stdout)
+
+    data: List[Dict[str, Any]] = json.loads(proc_items.stdout)
+
+    return data
 
 
-def add_ssh_keys(session, items, keyname):
+def add_ssh_keys(session: str, items: List[Dict[str, Any]], keyname: str) -> None:
     """
     Function to attempt to get keys from a vault item
     """
@@ -145,9 +149,9 @@ def add_ssh_keys(session, items, keyname):
         except IndexError:
             logging.warning('No "%s" field found for item %s', keyname, item['name'])
             continue
-        except KeyError as e:
+        except KeyError as error:
             logging.debug(
-                'No key "%s" found in item %s - skipping', e.args[0], item['name']
+                'No key "%s" found in item %s - skipping', error.args[0], item['name']
             )
             continue
         logging.debug('Private key file declared')
@@ -173,7 +177,7 @@ def add_ssh_keys(session, items, keyname):
             logging.warning('Could not add key to the SSH agent')
 
 
-def ssh_add(session, item_id, key_id):
+def ssh_add(session: str, item_id: str, key_id: str) -> None:
     """
     Function to get the key contents from the Bitwarden vault
     """
@@ -213,7 +217,7 @@ def ssh_add(session, item_id, key_id):
 
 if __name__ == '__main__':
 
-    def parse_args():
+    def parse_args() -> argparse.Namespace:
         """
         Function to parse command line arguments
         """
@@ -239,7 +243,7 @@ if __name__ == '__main__':
 
         return parser.parse_args()
 
-    def main():
+    def main() -> None:
         """
         Main program logic
         """
@@ -266,9 +270,9 @@ if __name__ == '__main__':
 
             logging.info('Attempting to add keys to ssh-agent')
             add_ssh_keys(session, items, args.customfield)
-        except subprocess.CalledProcessError as e:
-            if e.stderr:
-                logging.error('`%s` error: %s', e.cmd[0], e.stderr)
-            logging.debug('Error running %s', e.cmd)
+        except subprocess.CalledProcessError as error:
+            if error.stderr:
+                logging.error('`%s` error: %s', error.cmd[0], error.stderr)
+            logging.debug('Error running %s', error.cmd)
 
     main()