diff --git a/readthedocs/doc_builder/environments.py b/readthedocs/doc_builder/environments.py
index f0118164c2f..d142f897a7d 100644
--- a/readthedocs/doc_builder/environments.py
+++ b/readthedocs/doc_builder/environments.py
@@ -194,7 +194,7 @@ def sanitize_output(self, output):
             # https://code.djangoproject.com/ticket/28201
             sanitized = sanitized.replace('\x00', '')
         except (TypeError, AttributeError):
-            sanitized = None
+            sanitized = ""
 
         # Chunk the output data to be less than ``DATA_UPLOAD_MAX_MEMORY_SIZE``
         output_length = len(output) if output else 0
@@ -315,10 +315,8 @@ def run(self):
                 cmd_stdout, cmd_stderr = out
             else:
                 cmd_stdout = out
-            # Docker returns None when there's no stderr/stdout
-            # so we need to convert that to a string.
-            self.output = self.sanitize_output(cmd_stdout or "")
-            self.error = self.sanitize_output(cmd_stderr or "")
+            self.output = self.sanitize_output(cmd_stdout)
+            self.error = self.sanitize_output(cmd_stderr)
             cmd_ret = client.exec_inspect(exec_id=exec_cmd['Id'])
             self.exit_code = cmd_ret['ExitCode']
 
diff --git a/readthedocs/rtd_tests/tests/test_doc_building.py b/readthedocs/rtd_tests/tests/test_doc_building.py
index 0004d5bc0ea..f1b2d570bad 100644
--- a/readthedocs/rtd_tests/tests/test_doc_building.py
+++ b/readthedocs/rtd_tests/tests/test_doc_building.py
@@ -255,7 +255,7 @@ def test_error_output(self):
         cmd = BuildCommand(['/bin/bash', '-c', 'echo -n FOOBAR 1>&2'])
         cmd.run()
         self.assertEqual(cmd.output, 'FOOBAR')
-        self.assertIsNone(cmd.error)
+        self.assertEqual(cmd.error, "")
 
     def test_sanitize_output(self):
         cmd = BuildCommand(['/bin/bash', '-c', 'echo'])