diff --git a/api/src/main/java/org/apache/commons/rdf/Language.java b/api/src/main/java/org/apache/commons/rdf/Language.java
index 03f328c..e76e16d 100644
--- a/api/src/main/java/org/apache/commons/rdf/Language.java
+++ b/api/src/main/java/org/apache/commons/rdf/Language.java
@@ -36,7 +36,7 @@
         if ((id == null) || (id.equals(""))) {
             throw new IllegalArgumentException("A language id may not be null or empty");
         }
-        this.id = id;
+        this.id = id.toLowerCase();
     }
 
     @Override