diff --git a/src/liblzma/common/index.c b/src/liblzma/common/index.c
index 26e4e519..007e1570 100644
--- a/src/liblzma/common/index.c
+++ b/src/liblzma/common/index.c
@@ -825,8 +825,8 @@ lzma_index_cat(lzma_index *restrict dest, lzma_index *restrict src,
 				s->groups.root = &newg->node;
 			}
 
-			if (s->groups.rightmost == &g->node)
-				s->groups.rightmost = &newg->node;
+			assert(s->groups.rightmost == &g->node);
+			s->groups.rightmost = &newg->node;
 
 			lzma_free(g, allocator);