diff --git a/conservancy_beancount/errors.py b/conservancy_beancount/errors.py
index 3d9bdd2..486676a 100644
--- a/conservancy_beancount/errors.py
+++ b/conservancy_beancount/errors.py
@@ -14,24 +14,53 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
+import beancount.core.data as bc_data
+
 from typing import (
+    Any,
     Iterable,
+    Optional,
 )
 
+from .beancount_types import (
+    Directive,
+    MetaKey,
+    MetaValue,
+    Posting,
+    Transaction,
+    Type,
+)
+
+Meta = Optional[bc_data.Meta]
+
 class Error(Exception):
-    def __init__(self, message, entry, source=None):
+    def __init__(self,
+                 message: str,
+                 entry: Optional[Directive],
+                 source: Meta=None,
+    ) -> None:
         self.message = message
         self.entry = entry
-        self.source = entry.meta if source is None else source
+        if source:
+            self.source = source
+        elif entry is not None:
+            self.source = entry.meta
+        else:
+            self.source = {}
+            self._fill_source(self.source, '<unknown>')
 
-    def __repr__(self):
+    def __repr__(self) -> str:
         return "{clsname}<{source[filename]}:{source[lineno]}: {message}>".format(
             clsname=type(self).__name__,
             message=self.message,
             source=self.source,
         )
 
-    def _fill_source(self, source, filename='conservancy_beancount', lineno=0):
+    def _fill_source(self,
+                     source: bc_data.Meta,
+                     filename: str='conservancy_beancount',
+                     lineno: int=0,
+    ) -> None:
         source.setdefault('filename', filename)
         source.setdefault('lineno', lineno)
 
@@ -39,15 +68,27 @@ class Error(Exception):
 Iter = Iterable[Error]
 
 class BrokenLinkError(Error):
-    def __init__(self, txn, key, link, source=None):
+    def __init__(self,
+                 txn: Transaction,
+                 key: MetaKey,
+                 link: str,
+                 source: Meta=None,
+    ) -> None:
         super().__init__(
             "{} not found in repository: {}".format(key, link),
             txn,
             source,
         )
 
+
 class BrokenRTLinkError(Error):
-    def __init__(self, txn, key, link, parsed=True, source=None):
+    def __init__(self,
+                 txn: Transaction,
+                 key: MetaKey,
+                 link: str,
+                 parsed: Any=True,
+                 source: Meta=None,
+    ) -> None:
         if parsed:
             msg_fmt = "{} not found in RT: {}"
         else:
@@ -58,8 +99,13 @@ class BrokenRTLinkError(Error):
             source,
         )
 
+
 class ConfigurationError(Error):
-    def __init__(self, message, entry=None, source=None):
+    def __init__(self,
+                 message: str,
+                 entry: Optional[Directive]=None,
+                 source: Meta=None,
+    ) -> None:
         if source is None:
             source = {}
         self._fill_source(source)
@@ -67,7 +113,14 @@ class ConfigurationError(Error):
 
 
 class InvalidMetadataError(Error):
-    def __init__(self, txn, key, value=None, post=None, need_type=str, source=None):
+    def __init__(self,
+                 txn: Transaction,
+                 key: MetaKey,
+                 value: Optional[MetaValue]=None,
+                 post: Optional[bc_data.Posting]=None,
+                 need_type: Type=str,
+                 source: Meta=None,
+    ) -> None:
         if post is None:
             srcname = 'transaction'
         else:
diff --git a/conservancy_beancount/plugin/core.py b/conservancy_beancount/plugin/core.py
index 241d7f9..2107045 100644
--- a/conservancy_beancount/plugin/core.py
+++ b/conservancy_beancount/plugin/core.py
@@ -69,8 +69,9 @@ class Hook(Generic[Entry], metaclass=abc.ABCMeta):
     @abc.abstractmethod
     def run(self, entry: Entry) -> errormod.Iter: ...
 
-    def __init_subclass__(cls):
-        cls.DIRECTIVE = cls.__orig_bases__[0].__args__[0]
+    def __init_subclass__(cls) -> None:
+        # cls.__orig_bases__ comes from the ABCMeta metaclass
+        cls.DIRECTIVE = cls.__orig_bases__[0].__args__[0]  # type:ignore[attr-defined]
 
 
 TransactionHook = Hook[Transaction]
diff --git a/conservancy_beancount/plugin/meta_project.py b/conservancy_beancount/plugin/meta_project.py
index 343a100..ec221cb 100644
--- a/conservancy_beancount/plugin/meta_project.py
+++ b/conservancy_beancount/plugin/meta_project.py
@@ -31,6 +31,7 @@ from ..beancount_types import (
 from typing import (
     Any,
     Dict,
+    NoReturn,
     Optional,
     Set,
 )
@@ -43,7 +44,7 @@ class MetaProject(core._NormalizePostingMetadataHook):
     def __init__(self, config: configmod.Config, source_path: Path=PROJECT_DATA_PATH) -> None:
         repo_path = config.repository_path()
         if repo_path is None:
-            raise self._config_error("no repository configured")
+            self._config_error("no repository configured")
         project_data_path = repo_path / source_path
         source = {'filename': str(project_data_path)}
         try:
@@ -68,7 +69,7 @@ class MetaProject(core._NormalizePostingMetadataHook):
         else:
             self.VALUES_ENUM = core.MetadataEnum(self.METADATA_KEY, names, aliases)
 
-    def _config_error(self, msg: str, filename: Optional[Path]=None):
+    def _config_error(self, msg: str, filename: Optional[Path]=None) -> NoReturn:
         source = {}
         if filename is not None:
             source['filename'] = str(filename)
diff --git a/conservancy_beancount/rtutil.py b/conservancy_beancount/rtutil.py
index cb2dd37..2fa8c5f 100644
--- a/conservancy_beancount/rtutil.py
+++ b/conservancy_beancount/rtutil.py
@@ -196,11 +196,12 @@ class RT:
     # to be a method, it's just an internal decrator.
     def _cache_method(func: _URLLookup) -> _URLLookup:  # type:ignore[misc]
         @functools.wraps(func)
-        def caching_wrapper(self,
+        def caching_wrapper(self: 'RT',
                             ticket_id: RTId,
                             attachment_id: Optional[RTId]=None,
         ) -> Optional[str]:
-            cache_key = (str(ticket_id), attachment_id and str(attachment_id))
+            cache_key = (str(ticket_id),
+                         None if attachment_id is None else str(attachment_id))
             url: Optional[str]
             try:
                 url = self._cache[cache_key]
diff --git a/setup.cfg b/setup.cfg
index c7858f3..7ea8404 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -4,6 +4,7 @@ typecheck=pytest --addopts="--mypy conservancy_beancount"
 
 [mypy]
 disallow_any_unimported = True
+disallow_untyped_defs = True
 show_error_codes = True
 strict_equality = True
 warn_redundant_casts = True