diff --git a/src/main/java/com/datapipe/jenkins/vault/credentials/AbstractVaultTokenCredentialWithExpiration.java b/src/main/java/com/datapipe/jenkins/vault/credentials/AbstractVaultTokenCredentialWithExpiration.java
index 0cf695fe..cb330c87 100644
--- a/src/main/java/com/datapipe/jenkins/vault/credentials/AbstractVaultTokenCredentialWithExpiration.java
+++ b/src/main/java/com/datapipe/jenkins/vault/credentials/AbstractVaultTokenCredentialWithExpiration.java
@@ -44,13 +44,14 @@ public void setUsePolicies(Boolean usePolicies) {
         this.usePolicies = usePolicies;
     }
 
-    private transient Map<String, Calendar> tokenExpiry;
+    // Renamed from tokenExpiry to prevent XStream from attempting to deserialize old instances of this class which had the type `Calendar` prior to https://github.com/jenkinsci/hashicorp-vault-plugin/pull/223.
+    private transient Map<String, Calendar> tokenExpiryCache;
     private transient Map<String, String> tokenCache;
 
     protected AbstractVaultTokenCredentialWithExpiration(CredentialsScope scope, String id,
         String description) {
         super(scope, id, description);
-        tokenExpiry = new HashMap<>();
+        tokenExpiryCache = new HashMap<>();
         tokenCache = new HashMap<>();
     }
 
@@ -108,7 +109,7 @@ public Vault authorizeWithVault(VaultConfig config, List<String> policies) {
         // Upgraded instances can have these not initialized in the constructor (serialized jobs possibly)
         if (tokenCache == null) {
             tokenCache = new HashMap<>();
-            tokenExpiry = new HashMap<>();
+            tokenExpiryCache = new HashMap<>();
         }
 
         String cacheKey = getCacheKey(policies);
@@ -150,11 +151,11 @@ private void setTokenExpiry(Vault vault, String cacheKey) {
         }
         Calendar expiry = Calendar.getInstance();
         expiry.add(Calendar.SECOND, tokenTTL);
-        tokenExpiry.put(cacheKey, expiry);
+        tokenExpiryCache.put(cacheKey, expiry);
     }
 
     private boolean tokenExpired(String cacheKey) {
-        Calendar expiry = tokenExpiry.get(cacheKey);
+        Calendar expiry = tokenExpiryCache.get(cacheKey);
         if (expiry == null) {
             return true;
         }