diff --git a/src/main/org/apache/tools/ant/types/FilterSet.java b/src/main/org/apache/tools/ant/types/FilterSet.java
index 7269954a0..f69bf8bd8 100644
--- a/src/main/org/apache/tools/ant/types/FilterSet.java
+++ b/src/main/org/apache/tools/ant/types/FilterSet.java
@@ -245,6 +245,9 @@ public class FilterSet extends DataType {
         if (isReference()) {
             throw tooManyAttributes();
         }
+        if (startOfToken == null || "".equals(startOfToken)) {
+            throw new BuildException("beginToken must not be empty");
+        }
         this.startOfToken = startOfToken;
     }
 
@@ -265,6 +268,9 @@ public class FilterSet extends DataType {
         if (isReference()) {
             throw tooManyAttributes();
         }
+        if (endOfToken == null || "".equals(endOfToken)) {
+            throw new BuildException("endToken must not be empty");
+        }
         this.endOfToken = endOfToken;
     }