diff --git a/kwant/_common.py b/kwant/_common.py
index eda672a14e515a74e8bf5937a1a2ffb8d2dbc004..4cd0de1a3e6d4b084f3434bbfffef08871d51ded 100644
--- a/kwant/_common.py
+++ b/kwant/_common.py
@@ -41,30 +41,6 @@ class UserCodeError(Exception):
     pass
 
 
-class ExtensionUnavailable:
-    """Class that replaces unavailable extension modules in the Kwant namespace.
-
-    Some extensions for Kwant (e.g. 'kwant.continuum') require additional
-    dependencies that are not required for core functionality. When the
-    additional dependencies are not installed an instance of this class will
-    be inserted into Kwant's root namespace to simulate the presence of the
-    extension and inform users that they need to install additional
-    dependencies.
-
-    See https://mail.python.org/pipermail/python-ideas/2012-May/014969.html
-    for more details.
-    """
-
-    def __init__(self, name, dependencies):
-        self.name = name
-        self.dependencies = ', '.join(dependencies)
-
-    def __getattr__(self, _):
-        msg = ("'{}' is not available because one or more of the following "
-               "dependencies are not installed: {}")
-        raise RuntimeError(msg.format(self.name, self.dependencies))
-
-
 def ensure_isinstance(obj, typ, msg=None):
     if isinstance(obj, typ):
         return
diff --git a/kwant/continuum/__init__.py b/kwant/continuum/__init__.py
index 718f50151795830b73df1f9327aa591997d0cbdc..bc2420e9c7998af5cb7f3b33fe72555e778156d4 100644
--- a/kwant/continuum/__init__.py
+++ b/kwant/continuum/__init__.py
@@ -6,18 +6,15 @@
 # the file AUTHORS.rst at the top-level directory of this distribution and at
 # http://kwant-project.org/authors.
 
-import sys
-
-from .._common import ExtensionUnavailable
-
 try:
     from .discretizer import discretize, discretize_symbolic, build_discretized
     from ._common import sympify, lambdify
     from ._common import momentum_operators, position_operators
-except ImportError:
-    sys.modules[__name__] = ExtensionUnavailable(__name__, ('sympy',))
+except ImportError as error:
+    msg = ("'kwant.continuum' is not available because one or more of its "
+           "dependencies is not installed.")
+    raise ImportError(msg) from error
 
-del sys, ExtensionUnavailable
 
 __all__ = ['discretize', 'discretize_symbolic', 'build_discretized',
            'sympify', 'lambdify']