xforms-submit-error dispatched in response to HTTP 200 OK server response

A form has an xf:submission element with the following form:

<xf:submission replace="none" method="post" mode="synchronous" action="ex:action" ref="some/xpath/reference" id="i">
  <xf:action ev:event="xforms-submit-error">
    <!-- Other action components here... -->
    <xf:message>error</xf:message>
  </xf:action>
</xf:submission>

When I submit this form, the server logs indicate that ex:action is returning a "HTTP/1.1 200 OK" response, but the "error" message is displayed, indicating that the xforms-submit-error event is being dispatched.