diff --git a/contrib/groff/tmac/doc-common b/contrib/groff/tmac/doc-common
index 08dbb7d4019..958a4f06e32 100644
--- a/contrib/groff/tmac/doc-common
+++ b/contrib/groff/tmac/doc-common
@@ -41,6 +41,7 @@
 .nr %A 1
 .nr %B 1
 .nr %D 1
+.nr %I 1
 .nr %J 1
 .nr %N 1
 .nr %O 1