diff --git a/src/regexp_bt.c b/src/regexp_bt.c
index 9da345f..5452dda 100644
--- a/src/regexp_bt.c
+++ b/src/regexp_bt.c
@@ -5570,7 +5570,7 @@
       case ZREF + 7:
       case ZREF + 8:
       case ZREF + 9:
-	bulen += sprintf(buf + buflen, "ZREF%d", OP(op) - ZREF);
+	buflen += sprintf(buf + buflen, "ZREF%d", OP(op) - ZREF);
 	p = NULL;
 	break;
 #endif