diff --git a/masque/shapes/rect_collection.py b/masque/shapes/rect_collection.py
index eaf028f..291f23a 100644
--- a/masque/shapes/rect_collection.py
+++ b/masque/shapes/rect_collection.py
@@ -187,7 +187,12 @@ class RectCollection(Shape):
     def rotate(self, theta: float) -> Self:
         quarter_turns = int(numpy.rint(theta / (pi / 2)))
         if not numpy.isclose(theta, quarter_turns * (pi / 2)):
-            raise PatternError('RectCollection only supports Manhattan rotations')
+            raise PatternError(
+                f'RectCollection cannot rotate by {theta!r} radians; only Manhattan rotations (multiples of pi/2) are supported. '
+                'Explicitly replace the collection with to_polygons(), or call Pattern.polygonize() '
+                'on the pattern containing it (including referenced child patterns) before transforming, '
+                'flattening, or computing hierarchical bounds.'
+                )
         turns = quarter_turns % 4
         if turns == 0 or self._rects.size == 0:
             return self