diff --git a/src/regexp_nfa.c b/src/regexp_nfa.c
index cc29a4c..406a0e4 100644
--- a/src/regexp_nfa.c
+++ b/src/regexp_nfa.c
@@ -6678,7 +6678,9 @@
 		int subidx;
 		int bytelen;
 
-		if (t->state->c <= NFA_BACKREF9)
+#ifdef FEAT_SYN_HL
+		if (t->state->c >= NFA_BACKREF1 && t->state->c <= NFA_BACKREF9)
+#endif
 		{
 		    subidx = t->state->c - NFA_BACKREF1 + 1;
 		    result = match_backref(&t->subs.norm, subidx, &bytelen);