diff --git a/easypqp/convert.py b/easypqp/convert.py
index fc69712..ec821d8 100755
--- a/easypqp/convert.py
+++ b/easypqp/convert.py
@@ -307,7 +307,7 @@ def get(self):
 	def match_unimod(self, unimod):
 		def match_modifications(um, peptide):
 			monomeric_masses = {"A": 71.03711, "R": 156.10111, "N": 114.04293, "D": 115.02694, "C": 103.00919, "E": 129.04259, "Q": 128.05858, "G": 57.02146, "H": 137.05891, "I": 113.08406, "L": 113.08406, "K": 128.09496, "M": 131.04049, "F": 147.06841, "P": 97.05276, "S": 87.03203, "T": 101.04768, "W": 186.07931, "Y": 163.06333, "V": 99.06841,
-								'U': 150.95363, 'O': 237.14773}
+								'U': 150.95363, 'O': 237.14773, 'B': 0, 'J': 0, 'X': 0, 'Z': 0}
 			modified_peptide = peptide['peptide_sequence']
 
 			# parse terminal modifications