diff --git a/fastg2protlib/fastg2protlib.py b/fastg2protlib/fastg2protlib.py
index 3434172..6f849ff 100644
--- a/fastg2protlib/fastg2protlib.py
+++ b/fastg2protlib/fastg2protlib.py
@@ -305,7 +305,7 @@ def write_peptide_fasta(db, fasta_name="peptide.fasta"):
 
 
 def peptides_for_fastg(
-    fastg_filename,
+    fastg_filename=None,
     cleavage="trypsin",
     min_protein_length=166,
     min_peptide_length=10,
@@ -350,6 +350,10 @@ def peptides_for_fastg(
     """
     # endregion
 
+    if not fastg_filename:
+        print("A FASTG file path is mandatory.")
+        exit(0)
+
     logger.info(f"FASTG file: {fastg_filename}")
     logger.info(f"Protein len: {min_protein_length}")
     logger.info(f"Peptide len: {min_peptide_length}")