diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/Certificate.java b/acme4j-client/src/main/java/org/shredzone/acme4j/Certificate.java
index 5816109a..05875d5c 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/Certificate.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/Certificate.java
@@ -167,9 +167,6 @@ public class Certificate extends AcmeResource {
         Session session = login.getSession();
 
         URL resUrl = session.resourceUrl(Resource.REVOKE_CERT);
-        if (resUrl == null) {
-            throw new AcmeException("Server does not allow certificate revocation");
-        }
 
         try (Connection conn = session.connect()) {
             JSONBuilder claims = new JSONBuilder();
@@ -204,9 +201,6 @@ public class Certificate extends AcmeResource {
         LOG.debug("revoke using the domain key pair");
 
         URL resUrl = session.resourceUrl(Resource.REVOKE_CERT);
-        if (resUrl == null) {
-            throw new AcmeException("Server does not allow certificate revocation");
-        }
 
         try (Connection conn = session.connect()) {
             JSONBuilder claims = new JSONBuilder();
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/Challenge.java b/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/Challenge.java
index 6e0bf638..fcb090d9 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/Challenge.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/Challenge.java
@@ -128,7 +128,7 @@ public class Challenge extends AcmeJsonResource {
         }
 
         String loc = json.get(KEY_URL).asString();
-        if (loc != null && !loc.equals(getLocation().toString())) {
+        if (!loc.equals(getLocation().toString())) {
             throw new AcmeProtocolException("challenge has changed its location");
         }
 
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java b/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
index f705f5aa..1c6d54a0 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
@@ -13,8 +13,9 @@
  */
 package org.shredzone.acme4j.toolbox;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
+
 import java.io.IOException;
-import java.io.UnsupportedEncodingException;
 import java.io.Writer;
 import java.net.IDN;
 import java.net.URI;
@@ -99,9 +100,9 @@ public final class AcmeUtils {
     public static byte[] sha256hash(String z) {
         try {
             MessageDigest md = MessageDigest.getInstance("SHA-256");
-            md.update(z.getBytes("UTF-8"));
+            md.update(z.getBytes(UTF_8));
             return md.digest();
-        } catch (NoSuchAlgorithmException | UnsupportedEncodingException ex) {
+        } catch (NoSuchAlgorithmException ex) {
             throw new AcmeProtocolException("Could not compute hash", ex);
         }
     }
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/JSON.java b/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/JSON.java
index 237e79a9..4742f5f8 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/JSON.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/JSON.java
@@ -13,6 +13,7 @@
  */
 package org.shredzone.acme4j.toolbox;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
 import static java.util.stream.Collectors.joining;
 import static org.shredzone.acme4j.toolbox.AcmeUtils.parseTimestamp;
 
@@ -27,7 +28,6 @@ import java.net.MalformedURLException;
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.net.URL;
-import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 import java.time.Instant;
 import java.util.Collections;
@@ -96,7 +96,7 @@ public final class JSON implements Serializable {
      * @return {@link JSON} of the read content.
      */
     public static JSON parse(InputStream in) throws IOException {
-        try (BufferedReader reader = new BufferedReader(new InputStreamReader(in, "utf-8"))) {
+        try (BufferedReader reader = new BufferedReader(new InputStreamReader(in, UTF_8))) {
             String json = reader.lines().map(String::trim).collect(joining());
             return parse(json);
         }
@@ -347,7 +347,7 @@ public final class JSON implements Serializable {
             required();
             try {
                 byte[] raw = AcmeUtils.base64UrlDecode(val.toString());
-                return new JSON(path, JsonUtil.parseJson(new String(raw, StandardCharsets.UTF_8)));
+                return new JSON(path, JsonUtil.parseJson(new String(raw, UTF_8)));
             } catch (IllegalArgumentException | JoseException ex) {
                 throw new AcmeProtocolException(path + ": expected an encoded object", ex);
             }
@@ -494,7 +494,7 @@ public final class JSON implements Serializable {
 
         @Override
         public boolean equals(Object obj) {
-            if (obj == null || !(obj instanceof Value)) {
+            if (!(obj instanceof Value)) {
                 return false;
             }
             return Objects.equals(val, ((Value) obj).val);
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/TestUtils.java b/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/TestUtils.java
index 3ff0a718..67cac425 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/TestUtils.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/TestUtils.java
@@ -13,6 +13,7 @@
  */
 package org.shredzone.acme4j.toolbox;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
 import static java.util.Collections.unmodifiableList;
 import static java.util.stream.Collectors.toList;
 
@@ -319,7 +320,7 @@ public final class TestUtils {
         final JsonWebKey jwk = JsonWebKey.Factory.newJwk(keyPair.getPublic());
         Map<String, Object> params = new TreeMap<>(jwk.toParams(OutputControlLevel.PUBLIC_ONLY));
         MessageDigest md = MessageDigest.getInstance("SHA-256");
-        md.update(JsonUtil.toJson(params).getBytes("UTF-8"));
+        md.update(JsonUtil.toJson(params).getBytes(UTF_8));
         byte[] thumbprint = md.digest();
 
         System.out.println("N = " + params.get("n"));
diff --git a/acme4j-utils/src/main/java/org/shredzone/acme4j/util/CSRBuilder.java b/acme4j-utils/src/main/java/org/shredzone/acme4j/util/CSRBuilder.java
index d53bf0d7..74968244 100644
--- a/acme4j-utils/src/main/java/org/shredzone/acme4j/util/CSRBuilder.java
+++ b/acme4j-utils/src/main/java/org/shredzone/acme4j/util/CSRBuilder.java
@@ -13,6 +13,7 @@
  */
 package org.shredzone.acme4j.util;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
 import static java.util.Objects.requireNonNull;
 import static java.util.stream.Collectors.joining;
 import static org.shredzone.acme4j.toolbox.AcmeUtils.toAce;
@@ -311,7 +312,7 @@ public class CSRBuilder {
      *            is closed after use.
      */
     public void write(OutputStream out) throws IOException {
-        write(new OutputStreamWriter(out, "utf-8"));
+        write(new OutputStreamWriter(out, UTF_8));
     }
 
     @Override