diff --git a/RELEASES.md b/RELEASES.md
index f08873cb..9c453eb1 100644
--- a/RELEASES.md
+++ b/RELEASES.md
@@ -43,6 +43,11 @@ Linker
     entities within the XML document.  In some cases, it can reduce peak
     memory usage by half.
 
+Tools
+=====
+- `build-aux/check-coupling` will now prevent `supplier/` packages from
+  importing `ui/` packages; previously only the reverse was true.
+
 
 v18.0.3 (2021-07-21)
 ====================
diff --git a/build-aux/check-coupling b/build-aux/check-coupling
index 8051d8a8..a21a2494 100755
--- a/build-aux/check-coupling
+++ b/build-aux/check-coupling
@@ -70,13 +70,13 @@ find-violations()
             $( non-supplier-maps ) \
     | grep ' /suppliers/'
 
-  # Suppliers must not import other suppliers.
+  # Suppliers must not import other suppliers or UI packages.
   # TODO: Check against supplier maps
   for supplier in suppliers/*.xml; do
     local name=$( basename "$supplier" .xml )
 
     lsimports "$supplier" $( supplier-packages "$name" ) \
-      | grep ' /suppliers/' \
+      | grep ' /suppliers/\|/ui/' \
       | grep -v " /suppliers/$name/"
   done
 }