exception thrown after LTI 1.3 content selection - regression

I have an LTI 1.3 tool registered using deep linking/content selection. When I add a new activity using the tool, then Select Content, then select from the tool, I get an error from Moodle:

Exception - Argument 1 passed to params_to_string() must be an object, array given, called in [dirroot]/mod/lti/locallib.php on line 1576

The Apache log says:

PHP message: Default exception handler: Exception - Argument 1 passed to params_to_string() must be an object, array given, called in [dirroot]/mod/lti/locallib.php on line 1576 Debug: 
Error code: generalexceptionmessage

• line 1464 of /mod/lti/locallib.php: TypeError thrown
• line 1576 of /mod/lti/locallib.php: call to params_to_string()
• line 1629 of /mod/lti/locallib.php: call to content_item_to_form()
• line 84 of /mod/lti/contentitem_return.php: call to lti_tool_configuration_from_content_item()
  ', referer: https://[redacted]/

This did not happen in 4.0, only since switching to v4.1.1.