diff --git a/common/sourceTest/java/ch/systemsx/cisd/common/converter/ConverterPoolTest.java b/common/sourceTest/java/ch/systemsx/cisd/common/converter/ConverterPoolTest.java
index 57cd517ff0b88db4db6e2cc0bd661163af69ebc8..6896786a5c65403c806a8130c12ca4de1bce9963 100644
--- a/common/sourceTest/java/ch/systemsx/cisd/common/converter/ConverterPoolTest.java
+++ b/common/sourceTest/java/ch/systemsx/cisd/common/converter/ConverterPoolTest.java
@@ -29,7 +29,7 @@ import static org.testng.AssertJUnit.*;
 public final class ConverterPoolTest
 {
 
-    private static char NULL;
+    private static final char NULL = '\0';
 
     @Test
     public final void testRegisterConverter()