diff --git a/pkg/obiformats/fastseq_write_fasta.go b/pkg/obiformats/fastseq_write_fasta.go
index 88b029f..a79584e 100644
--- a/pkg/obiformats/fastseq_write_fasta.go
+++ b/pkg/obiformats/fastseq_write_fasta.go
@@ -90,6 +90,9 @@ func FormatFastaBatch(batch obiiter.BioSequenceBatch, formater FormatHeader, ski
 	log.Debugf("FormatFastaBatch: #%d : %d seqs", batch.Order(), batch.Len())
 
 	for _, seq := range batch.Slice() {
+		if len(seq.Id()) == 0 {
+			log.Fatalf("Sequence identifier is empty")
+		}
 		if seq.Len() > 0 {
 			// Write header directly into bs — no intermediate string
 			bs.WriteByte('>')
diff --git a/pkg/obiformats/fastseq_write_fastq.go b/pkg/obiformats/fastseq_write_fastq.go
index 845993a..679230c 100644
--- a/pkg/obiformats/fastseq_write_fastq.go
+++ b/pkg/obiformats/fastseq_write_fastq.go
@@ -64,6 +64,9 @@ func FormatFastqBatch(batch obiiter.BioSequenceBatch,
 	first := true
 
 	for _, seq := range batch.Slice() {
+		if len(seq.Id()) == 0 {
+			log.Fatalf("Sequence identifier is empty")
+		}
 		if seq.Len() > 0 {
 			_formatFastq(&bs, seq, formater)