diff --git a/log4j-jul/src/main/java/org/apache/logging/log4j/jul/LevelTranslator.java b/log4j-jul/src/main/java/org/apache/logging/log4j/jul/LevelTranslator.java
index c64c380..f601528 100644
--- a/log4j-jul/src/main/java/org/apache/logging/log4j/jul/LevelTranslator.java
+++ b/log4j-jul/src/main/java/org/apache/logging/log4j/jul/LevelTranslator.java
@@ -69,7 +69,9 @@ public final class LevelTranslator {
      * @return converted Level.
      */
     public static Level toLevel(final java.util.logging.Level level) {
-        return LEVEL_CONVERTER.toLevel(level);
+        Level log4jLevel = LEVEL_CONVERTER.toLevel(level);
+        if (log4jLevel == null) throw new IllegalArgumentException("Unknown java.util.logging.Level: " + level.toString());
+        return log4jLevel;
     }
 
     /**