diff --git a/dom/media/webspeech/synth/moz.build b/dom/media/webspeech/synth/moz.build
index de03c2fc4a4a..4a2eae08e783 100644
--- a/dom/media/webspeech/synth/moz.build
+++ b/dom/media/webspeech/synth/moz.build
@@ -41,8 +41,6 @@ if CONFIG['MOZ_WEBSPEECH']:
             'test/nsFakeSynthServices.cpp'
         ]
 
-    DIRS = []
-
     if CONFIG['MOZ_WIDGET_TOOLKIT'] == 'windows':
         DIRS += ['windows']
 
diff --git a/python/mozbuild/mozbuild/frontend/sandbox.py b/python/mozbuild/mozbuild/frontend/sandbox.py
index 0ae8e30daefb..9f2666f8c51b 100644
--- a/python/mozbuild/mozbuild/frontend/sandbox.py
+++ b/python/mozbuild/mozbuild/frontend/sandbox.py
@@ -286,6 +286,10 @@ class Sandbox(dict):
             if key in self._context and self._context[key] is not value:
                 raise KeyError('global_ns', 'reassign', key)
 
+            if (key not in self._context and isinstance(value, (list, dict))
+               and not value):
+                raise KeyError('Variable %s assigned an empty value.' % key)
+
             self._context[key] = value
         else:
             dict.__setitem__(self, key, value)
diff --git a/python/mozbuild/mozbuild/test/frontend/data/reader-error-empty-list/moz.build b/python/mozbuild/mozbuild/test/frontend/data/reader-error-empty-list/moz.build
new file mode 100644
index 000000000000..fc4ce0217978
--- /dev/null
+++ b/python/mozbuild/mozbuild/test/frontend/data/reader-error-empty-list/moz.build
@@ -0,0 +1,5 @@
+# -*- Mode: python; c-basic-offset: 4; indent-tabs-mode: nil; tab-width: 40 -*-
+# Any copyright is dedicated to the Public Domain.
+# http://creativecommons.org/publicdomain/zero/1.0/
+
+DIRS = []
diff --git a/python/mozbuild/mozbuild/test/frontend/test_reader.py b/python/mozbuild/mozbuild/test/frontend/test_reader.py
index 730588b71823..d50a0658354a 100644
--- a/python/mozbuild/mozbuild/test/frontend/test_reader.py
+++ b/python/mozbuild/mozbuild/test/frontend/test_reader.py
@@ -11,8 +11,10 @@ import unittest
 from mozunit import main
 
 from mozbuild.frontend.context import BugzillaComponent
-from mozbuild.frontend.reader import BuildReaderError
-from mozbuild.frontend.reader import BuildReader
+from mozbuild.frontend.reader import (
+    BuildReaderError,
+    BuildReader,
+)
 
 from mozbuild.test.common import MockConfig
 
@@ -246,6 +248,15 @@ class TestBuildReader(unittest.TestCase):
 
         contexts = list(reader.read_topsrcdir())
 
+    def test_error_empty_list(self):
+        reader = self.reader('reader-error-empty-list')
+
+        with self.assertRaises(BuildReaderError) as bre:
+            list(reader.read_topsrcdir())
+
+        e = bre.exception
+        self.assertIn('Variable DIRS assigned an empty value.', str(e))
+
     def test_inheriting_variables(self):
         reader = self.reader('inheriting-variables')