diff --git a/src/backends/llk.c b/src/backends/llk.c
index 2bc39daf92b371b3b22b783623442eee36053bc0..387dfd6a966b0f6a55004c8c23a14a03dd3830b0 100644
--- a/src/backends/llk.c
+++ b/src/backends/llk.c
@@ -84,6 +84,8 @@ static void *combine_entries(HHashSet *workset, void *dst, const void *src)
 // note: reuses parts of src to build dst!
 static void stringmap_merge(HHashSet *workset, HStringMap *dst, HStringMap *src)
 {
+  assert(src->arena == dst->arena);
+
   if(src->epsilon_branch) {
     if(dst->epsilon_branch)
       dst->epsilon_branch =