I'm trying to represent a non-natural amino acid Aib (2-aminoisobutyric acid) in ProForma. For the peptide A-Aib-L, I used:

seq = Peptidoform("A[MOD:01001]L")
seq.theoretical_mass

However, I get the following error:

TypeError: float() argument must be a string or a real number, not 'NoneType'
ModificationMassNotFoundError: Could not resolve the mass of (None, 1001) from Entity('MOD:01001', ...)

Analysis / possible bug:
[MOD:01001] is treated as modifying the preceding residue, not as an independent Aib.

MOD:01001 in PSI-MOD has DeltaMass=0, so Pyteomics cannot resolve its monoisotopic mass.

As a result, seq.theoretical_mass fails.

in PSI-MOD:

[Term]
id: MOD:01001
name: 2-aminoisobutyric acid residue (Aib)
def: "A protein modification that inserts or replaces a residue with a 2-aminoisobutyric acid." [DeltaMass:0]
comment: Modification from DeltaMass: Average Mass: 85.
synonym: "2-amino-2-methylpropanoic acid" EXACT PSI-MOD-alternate []
synonym: "2-amino-2-methylpropionic acid" EXACT PSI-MOD-alternate []
synonym: "2-methylalanine" EXACT PSI-MOD-alternate []
synonym: "Aib" EXACT PSI-MOD-alternate []
synonym: "alpha,alpha-dimethylglycine" EXACT PSI-MOD-alternate []
synonym: "alpha-aminoisobutyric acid" EXACT PSI-MOD-alternate []
synonym: "alpha-methylalanine" EXACT PSI-MOD-alternate []
property_value: Formula "C 4 H 7 N 1 O 1" xsd:string
property_value: MassAvg "85.11" xsd:float
property_value: MassMono "85.052764" xsd:float
property_value: Origin "X" xsd:string
property_value: Source "artifact" xsd:string
is_a: MOD:00850