diff --git a/NEWS b/NEWS
index be8de6682..4a573d91d 100644
--- a/NEWS
+++ b/NEWS
@@ -89,6 +89,10 @@ Bug fixes
 * Fix small OSS-Fuzz reported issues with CRAM encoding and long
   CIGARS and/or illegal positions. (PR #1775, PR #1801, PR #1817)
 
+* Fix issues with on-the-fly indexing of VCF/BCF (bcftools --write-index)
+  when not using multiple threads. (PR #1837. Fixes samtools/bcftools#2267,
+  reported by Giulio Genovese)
+
 * Stricter limits on POS / MPOS / TLEN in sam_parse1().  This fixes
   a signed overflow reported by OSS-Fuzz and should help prevent other
   as-yet undetected bugs. (PR #1812)
diff --git a/vcf.c b/vcf.c
index 7ce306f92..105c7539d 100644
--- a/vcf.c
+++ b/vcf.c
@@ -4238,6 +4238,8 @@ int vcf_write(htsFile *fp, const bcf_hdr_t *h, bcf1_t *v)
     if ( fp->format.compression!=no_compression ) {
         if (bgzf_flush_try(fp->fp.bgzf, fp->line.l) < 0)
             return -1;
+        if (fp->idx && !fp->fp.bgzf->mt)
+            hts_idx_amend_last(fp->idx, bgzf_tell(fp->fp.bgzf));
         ret = bgzf_write(fp->fp.bgzf, fp->line.s, fp->line.l);
     } else {
         ret = hwrite(fp->fp.hfile, fp->line.s, fp->line.l);