diff --git a/readthedocs/doc_builder/environments.py b/readthedocs/doc_builder/environments.py
index ea38b20c3d8..f0118164c2f 100644
--- a/readthedocs/doc_builder/environments.py
+++ b/readthedocs/doc_builder/environments.py
@@ -315,8 +315,10 @@ def run(self):
                 cmd_stdout, cmd_stderr = out
             else:
                 cmd_stdout = out
-            self.output = self.sanitize_output(cmd_stdout)
-            self.error = self.sanitize_output(cmd_stderr)
+            # 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 "")
             cmd_ret = client.exec_inspect(exec_id=exec_cmd['Id'])
             self.exit_code = cmd_ret['ExitCode']