diff --git a/log4j-over-slf4j/src/main/java/org/apache/log4j/Category.java b/log4j-over-slf4j/src/main/java/org/apache/log4j/Category.java
index fc8b50c20..5edde37f5 100644
--- a/log4j-over-slf4j/src/main/java/org/apache/log4j/Category.java
+++ b/log4j-over-slf4j/src/main/java/org/apache/log4j/Category.java
@@ -323,7 +323,8 @@ private int priorityToLevelInt(Priority p) {
         case Priority.FATAL_INT:
             return LocationAwareLogger.ERROR_INT;
         default:
-            throw new IllegalStateException("Unknown Priority " + p);
+            slf4jLogger.warn("Unknown Priority " + p);
+            return LocationAwareLogger.TRACE_INT;
         }
     }