diff --git a/tutor/commands/webui.py b/tutor/commands/webui.py
index b210bd7..2d8b4da 100644
--- a/tutor/commands/webui.py
+++ b/tutor/commands/webui.py
@@ -4,7 +4,7 @@ import platform
 import subprocess
 import sys
 import tarfile
-from typing import Any, Dict
+from typing import Any, Dict, Optional
 from urllib.request import urlopen
 
 import click
@@ -13,6 +13,7 @@ import click
 # the web ui can be launched even where there is no configuration.
 from .. import fmt
 from .. import env as tutor_env
+from .. import exceptions
 from .. import serialize
 from .context import Context
 
@@ -121,12 +122,17 @@ def check_gotty_binary(root: str) -> None:
     compressed.extract("./gotty", dirname)
 
 
-def load_config(root: str) -> Dict[str, Any]:
+def load_config(root: str) -> Dict[str, Optional[str]]:
     path = config_path(root)
     if not os.path.exists(path):
         save_webui_config_file(root, {"user": None, "password": None})
     with open(config_path(root)) as f:
-        return serialize.load(f)
+        config = serialize.load(f)
+    if not isinstance(config, dict):
+        raise exceptions.TutorError(
+            "Invalid webui: expected dict, got {}".format(config.__class__)
+        )
+    return config
 
 
 def save_webui_config_file(root: str, config: Dict[str, Any]) -> None:
diff --git a/tutor/config.py b/tutor/config.py
index 7e9def7..63d4e6c 100644
--- a/tutor/config.py
+++ b/tutor/config.py
@@ -1,5 +1,5 @@
 import os
-from typing import Dict, Any, Tuple
+from typing import cast, Dict, Any, Tuple
 
 from . import exceptions
 from . import env
@@ -59,12 +59,19 @@ def merge(
 
 
 def load_defaults() -> Dict[str, Any]:
-    return serialize.load(env.read_template_file("config.yml"))
+    config = serialize.load(env.read_template_file("config.yml"))
+    return cast(Dict[str, Any], config)
 
 
 def load_config_file(path: str) -> Dict[str, Any]:
     with open(path) as f:
-        return serialize.load(f.read())
+        config = serialize.load(f.read())
+    if not isinstance(config, dict):
+        raise exceptions.TutorError(
+            "Invalid configuration: expected dict, got {}".format(config.__class__)
+        )
+
+    return config
 
 
 def load_current(root: str, defaults: Dict[str, str]) -> Dict[str, Any]:
diff --git a/tutor/serialize.py b/tutor/serialize.py
index 9b29d9c..3831d93 100644
--- a/tutor/serialize.py
+++ b/tutor/serialize.py
@@ -1,5 +1,5 @@
 import re
-from typing import cast, Any, Dict, IO, Iterator, Tuple, Union
+from typing import Any, IO, Iterator, Tuple, Union
 
 import yaml
 from _io import TextIOWrapper
@@ -9,15 +9,15 @@ from yaml.scanner import ScannerError
 import click
 
 
-def load(stream: Union[str, IO[str]]) -> Dict[str, str]:
-    return cast(Dict[str, str], yaml.load(stream, Loader=yaml.SafeLoader))
+def load(stream: Union[str, IO[str]]) -> Any:
+    return yaml.load(stream, Loader=yaml.SafeLoader)
 
 
 def load_all(stream: str) -> Iterator[Any]:
     return yaml.load_all(stream, Loader=yaml.SafeLoader)
 
 
-def dump(content: Dict[str, str], fileobj: TextIOWrapper) -> None:
+def dump(content: Any, fileobj: TextIOWrapper) -> None:
     yaml.dump(content, stream=fileobj, default_flow_style=False)