diff --git a/src/util/treeMiscHelpers.js b/src/util/treeMiscHelpers.js
index dff02c07e..e46643fb0 100644
--- a/src/util/treeMiscHelpers.js
+++ b/src/util/treeMiscHelpers.js
@@ -173,6 +173,9 @@ export const getSeqChanges = (fromNode) => {
  */
 export const categoriseMutations = (mutations, observedMutations, seqChangesToRoot) => {
   const categorisedMutations = {};
+  if (typeof mutations !== "object") {
+    return categorisedMutations;
+  }
   for (const gene of Object.keys(mutations)) {
     const categories = { unique: [], homoplasies: [], gaps: [], reversionsToRoot: []};
     const isNuc = gene==="nuc";