diff --git a/build_modeller.py b/build_modeller.py
index 0cf7c56..3010a9f 100755
--- a/build_modeller.py
+++ b/build_modeller.py
@@ -46,7 +46,7 @@ class PDBModeler:
             trim_index = 1
         else:
             # 如果两条链都不需要裁剪，就直接返回原文件
-            return file
+            return None
 
         start, end = PDBModeler.find_non_dash_indices(seqs[trim_index].seq)
         
@@ -92,19 +92,19 @@ class PDBModeler:
         # save alignment in FASTA format
         aln.write(file=(self.outdir / 'alignment1.fasta').as_posix(), alignment_format='FASTA')
         slice_fasta = PDBModeler.align_sequences(self.outdir / 'alignment1.fasta')
-        slice_ali = self.outdir / 'alignment_slice.ali'
-        fx = pyfastx.Fasta(slice_fasta.as_posix(), build_index=True)
-        assert len(fx) == 1, "FASTA file should contain only one sequence"
-        PDBModeler.write_ali(slice_ali, fx[0].name, fx[0].seq)
-
-        env2 = Environ()
-        mdl2 = Model(env2, file=self.structure_file.as_posix(), model_segment=(f'FIRST:{self.chain}', f'LAST:{self.chain}'))
-        aln2 = Alignment(env2)
-        aln2.append_model(mdl2, align_codes=self.structure, atom_files=self.structure_file.as_posix())
-        aln2.append(file=slice_ali.as_posix(), align_codes=self.sequence)
-        aln2.align2d()
-        aln2.write(file=(self.outdir / 'alignment2.ali').as_posix(), alignment_format='PIR')
-        aln2.write(file=(self.outdir / 'alignment2.pap').as_posix(), alignment_format='PAP')
+        if slice_fasta:
+            slice_ali = self.outdir / 'alignment_slice.ali'
+            fx = pyfastx.Fasta(slice_fasta.as_posix(), build_index=True)
+            assert len(fx) == 1, "FASTA file should contain only one sequence"
+            PDBModeler.write_ali(slice_ali, fx[0].name, fx[0].seq)
+            env2 = Environ()
+            mdl2 = Model(env2, file=self.structure_file.as_posix(), model_segment=(f'FIRST:{self.chain}', f'LAST:{self.chain}'))
+            aln2 = Alignment(env2)
+            aln2.append_model(mdl2, align_codes=self.structure, atom_files=self.structure_file.as_posix())
+            aln2.append(file=slice_ali.as_posix(), align_codes=self.sequence)
+            aln2.align2d()
+            aln2.write(file=(self.outdir / 'alignment2.ali').as_posix(), alignment_format='PIR')
+            aln2.write(file=(self.outdir / 'alignment2.pap').as_posix(), alignment_format='PAP')
 
         log.verbose()