diff --git a/lib/DataIntegrityProof.js b/lib/DataIntegrityProof.js
index a6b6a01..f0d03d3 100644
--- a/lib/DataIntegrityProof.js
+++ b/lib/DataIntegrityProof.js
@@ -296,7 +296,7 @@ export class DataIntegrityProof extends LinkedDataProof {
         const previousProofs = Array.isArray(proof.previousProof) ?
           proof.previousProof : [proof.previousProof];
         if(!previousProofs.every(proofId =>
-          proofSet.some(proof => proof.id === proofId))) {
+          proofSet.some(proof => proof?.id === proofId))) {
           throw new Error(
             `Unable to find previousProof ${proof.previousProof}`);
         }