diff --git a/build.gradle b/build.gradle
index 1e1af3749..363446669 100644
--- a/build.gradle
+++ b/build.gradle
@@ -95,6 +95,10 @@ android {
         exclude 'META-INF/NOTICE'
         exclude 'META-INF/NOTICE.txt'
     }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_6
+        targetCompatibility JavaVersion.VERSION_1_6
+    }
 }
 
 check.dependsOn 'checkstyle'
diff --git a/src/com/fsck/k9/controller/MessagingController.java b/src/com/fsck/k9/controller/MessagingController.java
index 71ebb9ebb..918e1b152 100644
--- a/src/com/fsck/k9/controller/MessagingController.java
+++ b/src/com/fsck/k9/controller/MessagingController.java
@@ -5118,7 +5118,7 @@ public class MessagingController implements Runnable {
                     publicNotification.setContentText(formattedSender);
                 } else {
                     // Use a LinkedHashSet so that we preserve ordering (newest to oldest), but still remove duplicates
-                    Set<CharSequence> senders = new LinkedHashSet<>(NUM_SENDERS_IN_LOCK_SCREEN_NOTIFICATION);
+                    Set<CharSequence> senders = new LinkedHashSet<CharSequence>(NUM_SENDERS_IN_LOCK_SCREEN_NOTIFICATION);
                     for (Message message : messages) {
                         senders.add(getMessageSender(context, account, message));
                         if (senders.size() == NUM_SENDERS_IN_LOCK_SCREEN_NOTIFICATION) {
diff --git a/src/com/fsck/k9/preferences/GlobalSettings.java b/src/com/fsck/k9/preferences/GlobalSettings.java
index 88e0e2a3f..b12c2852c 100644
--- a/src/com/fsck/k9/preferences/GlobalSettings.java
+++ b/src/com/fsck/k9/preferences/GlobalSettings.java
@@ -252,7 +252,7 @@ public class GlobalSettings {
                 new V(32, new BooleanSetting(false))
             ));
         s.put("lockScreenNotificationVisibility", Settings.versions(
-            new V(37, new EnumSetting<>(LockScreenNotificationVisibility.class, LockScreenNotificationVisibility.MESSAGE_COUNT))
+            new V(37, new EnumSetting<LockScreenNotificationVisibility>(LockScreenNotificationVisibility.class, LockScreenNotificationVisibility.MESSAGE_COUNT))
         ));
 
         SETTINGS = Collections.unmodifiableMap(s);