Index: /FCKeditor.Java/trunk/java-core/src/main/java/net/fckeditor/FCKeditor.java
===================================================================
--- /FCKeditor.Java/trunk/java-core/src/main/java/net/fckeditor/FCKeditor.java	(revision 2790)
+++ /FCKeditor.Java/trunk/java-core/src/main/java/net/fckeditor/FCKeditor.java	(revision 2791)
@@ -125,6 +125,10 @@
 	 * @param instanceName
 	 *            name
+	 *           @throws IllegalArgumentException
 	 */
 	public void setInstanceName(final String instanceName) {
+		if (Utils.isBlank(instanceName))
+			throw new IllegalArgumentException(
+					"instanceName must be a valid HTML id");
 		this.instanceName = instanceName;
 	}