diff --git a/CHANGES b/CHANGES
index b5d984306..929b468fd 100644
--- a/CHANGES
+++ b/CHANGES
@@ -17,6 +17,7 @@ Bugs fixed
 ----------
 
 * #5460: html search does not work with some 3rd party themes
+* #5614: autodoc: incremental build is broken when builtin modules are imported
 
 Testing
 --------
diff --git a/sphinx/util/__init__.py b/sphinx/util/__init__.py
index 109b098ce..9bc88c82f 100644
--- a/sphinx/util/__init__.py
+++ b/sphinx/util/__init__.py
@@ -301,7 +301,9 @@ def get_module_source(modname):
             raise PycodeError('error getting filename for %r' % filename, err)
     if filename is None and loader:
         try:
-            return 'string', loader.get_source(modname)
+            filename = loader.get_source(modname)
+            if filename:
+                return 'string', filename
         except Exception as err:
             raise PycodeError('error getting source for %r' % modname, err)
     if filename is None:
diff --git a/tests/test_util.py b/tests/test_util.py
index 8e4fbd6b0..a3745e7a1 100644
--- a/tests/test_util.py
+++ b/tests/test_util.py
@@ -12,9 +12,14 @@
 import pytest
 from mock import patch
 
+from six import PY2
+
+import sphinx
+from sphinx.errors import PycodeError
 from sphinx.testing.util import strip_escseq
 from sphinx.util import (
-    display_chunk, encode_uri, parselinenos, status_iterator, xmlname_checker
+    display_chunk, encode_uri, get_module_source, parselinenos, status_iterator,
+    xmlname_checker
 )
 from sphinx.util import logging
 
@@ -42,6 +47,19 @@ def test_display_chunk():
     assert display_chunk(('hello', 'sphinx', 'world')) == 'hello .. world'
 
 
+def test_get_module_source():
+    if PY2:
+        assert get_module_source('sphinx') == ('file', sphinx.__file__.replace('.pyc', '.py'))
+    else:
+        assert get_module_source('sphinx') == ('file', sphinx.__file__)
+
+    # failed to obtain source information from builtin modules
+    with pytest.raises(PycodeError):
+        get_module_source('builtins')
+    with pytest.raises(PycodeError):
+        get_module_source('itertools')
+
+
 @pytest.mark.sphinx('dummy')
 @patch('sphinx.util.console._tw', 40)  # terminal width = 40
 def test_status_iterator(app, status, warning):