Originally reported by Jakub Wilk as Debian #1018965.

Reproducer:

$ cat ATTIC/IP-UR.man
.TH
.IP 1. 4
.UR https://bar.example
.UE

A workaround is not to leave the text empty, though that is supported (and used by groff's own man pages, in grotty(1) and pic(1)).

It won't do to just blindly stick a <td> in front of the link text in this case:

diff --git a/tmac/an.tmac b/tmac/an.tmac
index 33e88be62..732eb026a 100644
--- a/tmac/an.tmac
+++ b/tmac/an.tmac
@@ -1105,8 +1105,9 @@ contains unsupported escape sequence
 .    \" If there was no link text, format URI as its own link text.  We
 .    \" don't add angle brackets here.
 .    el \{\
+.      an-warn GBR: no URI
 .      if \\n[an*is-output-html] \
-.        nop \X^html:<a href="\\*[an*prefix]\\*[an*hyperlink]">\
+.        nop \X^html:<td><a href="\\*[an*prefix]\\*[an*hyperlink]">\
 \\*[an*hyperlink]</a>^\c
 .      if \\n[an*is-output-terminal] \
 .        nop \X^tty: link \\*[an*prefix]\\*[an*hyperlink]^\

...since that introduces spurious `td` tags into URIs without linked text that are not in "lists" (indented paragraphs).

My surmise is that the state machine grohtml uses has gotten confused.