diff --git a/sphinx/application.py b/sphinx/application.py
index 630bff36f..5f6b92f50 100644
--- a/sphinx/application.py
+++ b/sphinx/application.py
@@ -124,6 +124,7 @@ class Sphinx(object):
         self.config = Config(confdir, CONFIG_FILENAME,
                              confoverrides or {}, self.tags)
         self.config.check_unicode(self.warn)
+        # defer checking types until i18n has been initialized
 
         # set confdir to srcdir if -C given (!= no confdir); a few pieces
         # of code expect a confdir to be set
@@ -172,6 +173,8 @@ class Sphinx(object):
 
         # set up translation infrastructure
         self._init_i18n()
+        # check all configuration values for permissible types
+        self.config.check_types(self.warn)
         # set up the build environment
         self._init_env(freshenv)
         # set up the builder
diff --git a/sphinx/config.py b/sphinx/config.py
index cf6bed087..be30d1ea4 100644
--- a/sphinx/config.py
+++ b/sphinx/config.py
@@ -14,7 +14,7 @@ from os import path
 
 from six import PY3, iteritems, string_types, binary_type, integer_types
 
-from sphinx.errors import ConfigError
+from sphinx.errors import ConfigError, ConfigWarning
 from sphinx.locale import l_
 from sphinx.util.osutil import make_filename, cd
 from sphinx.util.pycompat import execfile_
@@ -249,6 +249,30 @@ class Config(object):
         self.setup = config.get('setup', None)
         self.extensions = config.get('extensions', [])
 
+    def check_types(self, warn):
+        # check all values for deviation from the default value's type, since
+        # that can result in TypeErrors all over the place
+        # NB. since config values might use l_() we have to wait with calling
+        # this method until i18n is initialized
+        for name in self._raw_config:
+            if name not in Config.config_values:
+                continue  # we don't know a default value
+            default, dummy_rebuild = Config.config_values[name]
+            if hasattr(default, '__call__'):
+                default = default(self)  # could invoke l_()
+            if default is None:
+                continue
+            current = self[name]
+            if type(current) is type(default):
+                continue
+            common_bases = (
+                set(type(current).__bases__) & set(type(default).__bases__))
+            common_bases.discard(object)
+            if common_bases:
+                continue  # at least we share a non-trivial base class
+            warn("the config value %r has type `%s', defaults to `%s.'"
+                    % (name, type(current).__name__, type(default).__name__))
+
     def check_unicode(self, warn):
         # check all string values for non-ASCII characters in bytestrings,
         # since that can result in UnicodeErrors all over the place
@@ -296,7 +320,6 @@ class Config(object):
         for name in config:
             if name in self.values:
                 self.__dict__[name] = config[name]
-        del self._raw_config
 
     def __getattr__(self, name):
         if name.startswith('_'):
diff --git a/sphinx/errors.py b/sphinx/errors.py
index 3d7a5eb47..4e9828dc2 100644
--- a/sphinx/errors.py
+++ b/sphinx/errors.py
@@ -3,8 +3,8 @@
     sphinx.errors
     ~~~~~~~~~~~~~
 
-    Contains SphinxError and a few subclasses (in an extra module to avoid
-    circular import problems).
+    Contains SphinxError, a few subclasses (in an extra module to avoid
+    circular import problems), and related classes.
 
     :copyright: Copyright 2007-2014 by the Sphinx team, see AUTHORS.
     :license: BSD, see LICENSE for details.
@@ -75,3 +75,9 @@ class SphinxParallelError(Exception):
     def __str__(self):
         return traceback.format_exception_only(
             self.orig_exc.__class__, self.orig_exc)[0].strip()
+
+class ConfigWarning(UserWarning):
+    """
+    Base category for warnings about dubious configuration values.
+    """
+    pass
diff --git a/tests/test_config.py b/tests/test_config.py
index 0dcf3fa3e..c11c07219 100644
--- a/tests/test_config.py
+++ b/tests/test_config.py
@@ -133,3 +133,11 @@ def test_config_eol(tmpdir):
         cfg = Config(tmpdir, 'conf.py', {}, None)
         cfg.init_values(lambda warning: 1/0)
         assert cfg.project == u'spam'
+
+
+@with_app(confoverrides={'master_doc': 123})
+def test_check_types(app, status, warning):
+    # WARNING: the config value 'master_doc' has type `int', defaults to `str.'
+    assert any(buf.startswith('WARNING:')
+               and 'master_doc' in buf and 'int' in buf and 'str' in buf
+               for buf in warning.buflist)