diff --git a/src/main/org/apache/tools/ant/taskdefs/optional/ejb/GenericDeploymentTool.java b/src/main/org/apache/tools/ant/taskdefs/optional/ejb/GenericDeploymentTool.java
index 94127ec2f..4f6c57d36 100644
--- a/src/main/org/apache/tools/ant/taskdefs/optional/ejb/GenericDeploymentTool.java
+++ b/src/main/org/apache/tools/ant/taskdefs/optional/ejb/GenericDeploymentTool.java
@@ -574,8 +574,10 @@ public class GenericDeploymentTool implements EJBDeploymentTool {
 
             if (endBaseName != -1) {
                 baseName = descriptorFileName.substring(0, endBaseName);
+            } else {
+                throw new BuildException("Unable to determine jar name " 
+                    + "from descriptor \"" + descriptorFileName + "\"");
             }
-            baseName = descriptorFileName.substring(0, endBaseName);
         } else if (config.namingScheme.getValue().equals(EjbJar.NamingScheme.DIRECTORY)) {
             File descriptorFile = new File(config.descriptorDir, descriptorFileName);
             String path = descriptorFile.getAbsolutePath();