allow null value if operator is IS_SET or IS_NOT_SET
diff --git a/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/AbstractTextFilter.java b/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/AbstractTextFilter.java
index 226cc00..e92ddec 100644
--- a/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/AbstractTextFilter.java
+++ b/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/AbstractTextFilter.java
@@ -87,7 +87,7 @@
 					"This filter does not support the " + operator.toString()
 							+ " operation");
 		}
-		if (null == value){
+		if (null == value && operator != Operator.IS_SET && operator != Operator.IS_NOT_SET  ){
 			throw new IllegalArgumentException(
 					"Value can not be null");
 		}
diff --git a/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/EnumFilter.java b/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/EnumFilter.java
index 5f6f44d..637c39d 100644
--- a/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/EnumFilter.java
+++ b/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/EnumFilter.java
@@ -49,7 +49,7 @@
 		};
 		
 		// ensure that value1 is not null
-		if (null == value){
+		if (null == value && operator != Operator.IS_SET && operator != Operator.IS_NOT_SET  ){
 			throw new IllegalArgumentException(
 					"value1 can not be null");
 		}
diff --git a/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/NumberFilter.java b/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/NumberFilter.java
index 16fdcfc..437221d 100644
--- a/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/NumberFilter.java
+++ b/org.eclipse.rmf.reqif10.search/src/org/eclipse/rmf/reqif10/search/filter/NumberFilter.java
@@ -78,7 +78,7 @@
 		};
 		
 		// ensure that value1 is not null
-		if (null == value1){
+		if (null == value1 && operator != Operator.IS_SET && operator != Operator.IS_NOT_SET  ){
 			throw new IllegalArgumentException(
 					"value1 can not be null");
 		}