diff --git a/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/CryptElement.java b/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/CryptElement.java
index 1189d1607..7a9874f79 100644
--- a/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/CryptElement.java
+++ b/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/CryptElement.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2017 Florian Schmaus, 2018 Paul Schaub.
+ * Copyright 2017-2019 Florian Schmaus, 2018 Paul Schaub.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -32,11 +32,11 @@ public class CryptElement extends EncryptedOpenPgpContentElement {
 
     public static final String ELEMENT = "crypt";
 
-    public CryptElement(Set<Jid> to, String rpad, Date timestamp, List<ExtensionElement> payload) {
+    public CryptElement(Set<? extends Jid> to, String rpad, Date timestamp, List<ExtensionElement> payload) {
         super(to, rpad, timestamp, payload);
     }
 
-    public CryptElement(Set<Jid> to, List<ExtensionElement> payload) {
+    public CryptElement(Set<? extends Jid> to, List<ExtensionElement> payload) {
         super(to, payload);
     }
 
diff --git a/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/EncryptedOpenPgpContentElement.java b/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/EncryptedOpenPgpContentElement.java
index 0494e80d0..a79dc8997 100644
--- a/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/EncryptedOpenPgpContentElement.java
+++ b/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/EncryptedOpenPgpContentElement.java
@@ -38,14 +38,14 @@ public abstract class EncryptedOpenPgpContentElement extends OpenPgpContentEleme
 
     private final String rpad;
 
-    protected EncryptedOpenPgpContentElement(Set<Jid> to, String rpad, Date timestamp, List<ExtensionElement> payload) {
+    protected EncryptedOpenPgpContentElement(Set<? extends Jid> to, String rpad, Date timestamp, List<ExtensionElement> payload) {
         super(Objects.requireNonNullNorEmpty(
                 to, "Encrypted OpenPGP content elements must have at least one 'to' attribute."),
                 timestamp, payload);
         this.rpad = Objects.requireNonNull(rpad);
     }
 
-    protected EncryptedOpenPgpContentElement(Set<Jid> to, List<ExtensionElement> payload) {
+    protected EncryptedOpenPgpContentElement(Set<? extends Jid> to, List<ExtensionElement> payload) {
         super(Objects.requireNonNullNorEmpty(
                 to, "Encrypted OpenPGP content elements must have at least one 'to' attribute."),
                 new Date(), payload);
diff --git a/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/OpenPgpContentElement.java b/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/OpenPgpContentElement.java
index 2b61db669..db7014eef 100644
--- a/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/OpenPgpContentElement.java
+++ b/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/OpenPgpContentElement.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2017 Florian Schmaus, 2018 Paul Schaub.
+ * Copyright 2017-2019 Florian Schmaus, 2018 Paul Schaub.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -48,13 +48,13 @@ public abstract class OpenPgpContentElement implements ExtensionElement {
     public static final String ATTR_STAMP = "stamp";
     public static final String ELEM_PAYLOAD = "payload";
 
-    private final Set<Jid> to;
+    private final Set<? extends Jid> to;
     private final Date timestamp;
     private final MultiMap<String, ExtensionElement> payload;
 
     private String timestampString;
 
-    protected OpenPgpContentElement(Set<Jid> to, Date timestamp, List<ExtensionElement> payload) {
+    protected OpenPgpContentElement(Set<? extends Jid> to, Date timestamp, List<ExtensionElement> payload) {
         this.to = to;
         this.timestamp = Objects.requireNonNull(timestamp);
         this.payload = new MultiMap<>();
@@ -68,7 +68,7 @@ public abstract class OpenPgpContentElement implements ExtensionElement {
      *
      * @return recipients.
      */
-    public final Set<Jid> getTo() {
+    public final Set<? extends Jid> getTo() {
         return to;
     }
 
diff --git a/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/SigncryptElement.java b/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/SigncryptElement.java
index d0d165a86..c92f02f0f 100644
--- a/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/SigncryptElement.java
+++ b/smack-openpgp/src/main/java/org/jivesoftware/smackx/ox/element/SigncryptElement.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright 2017 Florian Schmaus, 2018 Paul Schaub.
+ * Copyright 2017-2019 Florian Schmaus, 2018 Paul Schaub.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -35,11 +35,11 @@ public class SigncryptElement extends EncryptedOpenPgpContentElement {
 
     public static final String ELEMENT = "signcrypt";
 
-    public SigncryptElement(Set<Jid> to, String rpad, Date timestamp, List<ExtensionElement> payload) {
+    public SigncryptElement(Set<? extends Jid> to, String rpad, Date timestamp, List<ExtensionElement> payload) {
         super(to, rpad, timestamp, payload);
     }
 
-    public SigncryptElement(Set<Jid> to, List<ExtensionElement> payload) {
+    public SigncryptElement(Set<? extends Jid> to, List<ExtensionElement> payload) {
         super(to, payload);
     }