diff --git a/htmlSanityCheck-core/src/main/java/org/aim42/htmlsanitycheck/check/CheckerCreator.java b/htmlSanityCheck-core/src/main/java/org/aim42/htmlsanitycheck/check/CheckerCreator.java
index 9f9b3297..ecf23f07 100644
--- a/htmlSanityCheck-core/src/main/java/org/aim42/htmlsanitycheck/check/CheckerCreator.java
+++ b/htmlSanityCheck-core/src/main/java/org/aim42/htmlsanitycheck/check/CheckerCreator.java
@@ -8,12 +8,16 @@
 import java.util.stream.Collectors;
 
 /**
- * abstract factory to create Checker instances
+ * Factory to create Checker instances
  */
 public class CheckerCreator {
     private static final Logger logger = LoggerFactory.getLogger(CheckerCreator.class);
 
-    public static List<Checker> createCheckerClassesFrom(final List<Class<? extends Checker>> checkerClasses, final Configuration configuration) {
+    private CheckerCreator() {
+    }
+
+    public static List<Checker> createCheckerClassesFrom(final List<Class<? extends Checker>> checkerClasses,
+                                                         final Configuration configuration) {
 
         return checkerClasses.stream()
                 .map(checkerClass -> CheckerCreator.createSingleChecker(checkerClass, configuration))
@@ -27,9 +31,13 @@ private static boolean isCase(Class<? extends Checker> caseValue, Class<? extend
         return false;
     }
 
-    public static Checker createSingleChecker(final Class<? extends Checker> checkerClass, final Configuration pConfig) {
-        Checker checker;
+    public static Checker createSingleChecker(final Class<? extends Checker> checkerClass,
+                                              final Configuration configuration) {
+        if (null == checkerClass) {
+            throw new UnknownCheckerException("Checker Class must not be 'null'");
+        }
 
+        Checker checker;
         // switch over all possible Checker classes
         // in case of new Checkers, this has to be adapted,
         // as Checker constructors will differ in minor details!
@@ -37,21 +45,21 @@ public static Checker createSingleChecker(final Class<? extends Checker> checker
         // clearly violates the open-close principle
 
         if (isCase(BrokenCrossReferencesChecker.class, checkerClass)) {
-            checker = new BrokenCrossReferencesChecker(pConfig);
+            checker = new BrokenCrossReferencesChecker(configuration);
         } else if (isCase(BrokenHttpLinksChecker.class, checkerClass)) {
-            checker = new BrokenHttpLinksChecker(pConfig);
+            checker = new BrokenHttpLinksChecker(configuration);
         } else if (isCase(DuplicateIdChecker.class, checkerClass)) {
-            checker = new DuplicateIdChecker(pConfig);
+            checker = new DuplicateIdChecker(configuration);
         } else if (isCase(ImageMapChecker.class, checkerClass)) {
-            checker = new ImageMapChecker(pConfig);
+            checker = new ImageMapChecker(configuration);
         } else if (isCase(MissingAltInImageTagsChecker.class, checkerClass)) {
-            checker = new MissingAltInImageTagsChecker(pConfig);
+            checker = new MissingAltInImageTagsChecker(configuration);
         } else if (isCase(MissingImageFilesChecker.class, checkerClass)) {
-            checker = new MissingImageFilesChecker(pConfig);
+            checker = new MissingImageFilesChecker(configuration);
         } else if (isCase(MissingLocalResourcesChecker.class, checkerClass)) {
-            checker = new MissingLocalResourcesChecker(pConfig);
+            checker = new MissingLocalResourcesChecker(configuration);
         } else {
-            logger.warn("unknown Checker " + checkerClass.toString());
+            logger.error("unknown Checker {}", checkerClass);
             throw new UnknownCheckerException(checkerClass.toString());
         }