https external-graphics

Hello,
I'm having some problems with getting images to resolve when using https instead of http. An example .fo file follows:
<?xml version="1.0" encoding="utf-8"?>
<fo:root xmlns:fo="http://www.w3.org/1999/XSL/Format">
	<fo:layout-master-set>
		<fo:simple-page-master page-width="8.5in" page-height="11in" master-name="BodyPage">
			<fo:region-body margin-top=".5in" margin-right=".5in" margin-bottom=".5in" margin-left=".5in"/>
			<fo:region-before region-name="header" extent=".5in" display-align="after"/>
			<fo:region-after region-name="footer" extent=".5in" display-align="before"/>
		</fo:simple-page-master>
	</fo:layout-master-set>
	<fo:page-sequence master-reference="BodyPage">
		<fo:flow flow-name="xsl-region-body">
			<fo:block space-before="1pc">
				<fo:external-graphic src="url(https://ichef.bbci.co.uk/images/ic/960x540/p01br4cp.jpg)" overflow="hidden" content-width="scale-to-fit" content-height="scale-to-fit" width="100%" scaling="uniform"/>
			</fo:block>
		</fo:flow>
	</fo:page-sequence>
</fo:root> 

When I run this:
java.exe -Djava.ext.dirs=C:\fop\fop-2.3 org.apache.fop.cli.Main -d -fo C:\foo\testhttps.fo -pdf C:\foo\testhttps.pdf

Despite curl being able to fetch this image (using https), I get this error:
SEVERE: Image not found. URI: https://ichef.bbci.co.uk/images/ic/960x540/p01br4cp.jpg. (See position 13:202)

But when I simply change https to http, things work. Is that expected? If so, why isn't https supported? Are there any workarounds?
Thanks in advance for any help/advice!

-Neil