diff --git a/src/nfa/castlecompile.cpp b/src/nfa/castlecompile.cpp
index b709d3c5..661c9c2c 100644
--- a/src/nfa/castlecompile.cpp
+++ b/src/nfa/castlecompile.cpp
@@ -772,7 +772,7 @@ bool mergeCastle(CastleProto &c1, const CastleProto &c2,
         const u32 top = m.first;
         const PureRepeat &pr = m.second;
         DEBUG_PRINTF("top %u\n", top);
-        u32 new_top = c1.add(pr);
+        u32 new_top = c1.merge(pr);
         top_map[top] = new_top;
         DEBUG_PRINTF("adding repeat: map %u->%u\n", top, new_top);
     }
diff --git a/src/nfa/castlecompile.h b/src/nfa/castlecompile.h
index aa4ed354..ea5f06da 100644
--- a/src/nfa/castlecompile.h
+++ b/src/nfa/castlecompile.h
@@ -128,7 +128,9 @@ buildCastle(const CastleProto &proto,
             const CompileContext &cc, const ReportManager &rm);
 
 /**
- * \brief Merge two CastleProto prototypes together, if possible.
+ * \brief Merge two CastleProto prototypes together, if possible. If a
+ * particular repeat from c2 is already in c1, then it will be reused rather
+ * than adding a duplicate repeat.
  *
  * Returns true if merge of all repeats in c2 into c1 succeeds, and fills
  * mapping with the repeat indices.