diff --git a/spectrum_io/spectral_library/digest.py b/spectrum_io/spectral_library/digest.py
index 3e15bd1..8afb444 100644
--- a/spectrum_io/spectral_library/digest.py
+++ b/spectrum_io/spectral_library/digest.py
@@ -85,7 +85,7 @@ def main(args):
 
 def valid_prosit_peptide(peptide: str) -> bool:
     """Check if peptide is valid."""
-    return len(peptide) <= 30 and "U" not in peptide and "X" not in peptide
+    return len(peptide) <= 30 and "U" not in peptide and "X" not in peptide and "*" not in peptide
 
 
 def parse_args(call):