diff --git a/code/gen_structure_graph.py b/code/gen_structure_graph.py
index d37c3dd..f63f87d 100644
--- a/code/gen_structure_graph.py
+++ b/code/gen_structure_graph.py
@@ -110,6 +110,11 @@ def cbeta_distance_matrix(pdb_fn, start=0, end=None):
     # loop through each residue and find the coordinates of cbeta
     for i, (residue_number, values) in enumerate(grouped):
 
+        # later versions of biopandas are returning the residue_number
+        # as a tuple, so convert it to a single integer
+        if isinstance(residue_number, tuple) and len(residue_number) == 1:
+            residue_number = residue_number[0]
+
         # skip residues not in the range
         end_index = (len(grouped) if end is None else end)
         if i not in range(start, end_index):