diff --git a/src/lj_strfmt_num.c b/src/lj_strfmt_num.c
index 79ec0263..c6e776aa 100644
--- a/src/lj_strfmt_num.c
+++ b/src/lj_strfmt_num.c
@@ -454,7 +454,8 @@ static char *lj_strfmt_wfnum(SBuf *sb, SFormat sf, lua_Number n, char *p)
 	    prec--;
 	    if (!i) {
 	      if (ndlo == ndhi) { prec = 0; break; }
-	      lj_strfmt_wuint9(tail, nd[++ndlo]);
+	      ndlo = (ndlo + 1) & 0x3f;
+	      lj_strfmt_wuint9(tail, nd[ndlo]);
 	      i = 9;
 	    }
 	  }