diff --git a/taxoniumtools/src/taxoniumtools/ushertools.py b/taxoniumtools/src/taxoniumtools/ushertools.py
index c7eb0eb3..1b3bede9 100644
--- a/taxoniumtools/src/taxoniumtools/ushertools.py
+++ b/taxoniumtools/src/taxoniumtools/ushertools.py
@@ -115,8 +115,8 @@ def get_mutations(past_nuc_muts_dict,
 
         initial_codon = [seq[gene_codon.positions[x]] for x in range(3)]
 
-        relevant_past_muts = [(gene_codon.positions[x], past_nuc_muts_dict[x])
-                              for x in gene_codon.positions
+        relevant_past_muts = [(x, past_nuc_muts_dict[x])
+                              for x in gene_codon.positions.values()
                               if x in past_nuc_muts_dict]
         flipped_dict = {
             position: offset