diff --git a/image.go b/image.go
index <HASH>..<HASH> 100644
--- a/image.go
+++ b/image.go
@@ -494,6 +494,9 @@ func (i *Image) DrawTriangles(vertices []Vertex, indices []uint16, img *Image, o
 	}
 
 	// TODO: Implement this.
+	if img.isSubImage() {
+		panic("using a subimage at DrawTriangles is not implemented")
+	}
 	if i.isSubimage() {
 		panic("render to a subimage is not implemented")
 	}
graphics: Forbid using a subimage at DrawTriangles (#<I>)