diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/util/ClaimBuilder.java b/acme4j-client/src/main/java/org/shredzone/acme4j/util/ClaimBuilder.java
index f554e79b..6fd9d685 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/util/ClaimBuilder.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/util/ClaimBuilder.java
@@ -73,7 +73,7 @@ public class ClaimBuilder {
      */
     public ClaimBuilder put(String key, Date value) {
         if (value == null) {
-            put(key, null);
+            put(key, (Object) null);
             return this;
         }
 
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/util/ClaimBuilderTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/util/ClaimBuilderTest.java
index 6b22cb7d..4187157c 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/util/ClaimBuilderTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/util/ClaimBuilderTest.java
@@ -20,6 +20,7 @@ import java.io.IOException;
 import java.security.KeyPair;
 import java.security.NoSuchAlgorithmException;
 import java.util.Calendar;
+import java.util.Date;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.TimeZone;
@@ -82,8 +83,9 @@ public class ClaimBuilderTest {
 
         ClaimBuilder cb = new ClaimBuilder();
         cb.put("fooDate", cal.getTime());
+        cb.put("fooNull", (Date) null);
 
-        assertThat(cb.toString(), is("{\"fooDate\":\"2016-06-01T03:13:46Z\"}"));
+        assertThat(cb.toString(), is("{\"fooDate\":\"2016-06-01T03:13:46Z\",\"fooNull\":null}"));
     }
 
     /**