diff --git a/jpa/plugins/org.eclipse.jpt.jpa.core/src/org/eclipse/jpt/jpa/core/internal/context/java/AbstractJavaPersistentType.java b/jpa/plugins/org.eclipse.jpt.jpa.core/src/org/eclipse/jpt/jpa/core/internal/context/java/AbstractJavaPersistentType.java
index cf94122..330804c 100644
--- a/jpa/plugins/org.eclipse.jpt.jpa.core/src/org/eclipse/jpt/jpa/core/internal/context/java/AbstractJavaPersistentType.java
+++ b/jpa/plugins/org.eclipse.jpt.jpa.core/src/org/eclipse/jpt/jpa/core/internal/context/java/AbstractJavaPersistentType.java
@@ -978,7 +978,9 @@
 	}
 
 	public void gatherRootStructureNodes(JpaFile jpaFile, Collection<JpaStructureNode> rootStructureNodes) {
-		if (this.getResource().equals(jpaFile.getFile())) {
+		IResource resource = this.getResource();
+		// the resource can be null if the resource type is "external"
+		if (resource != null && resource.equals(jpaFile.getFile())) {
 			rootStructureNodes.add(this);
 		}
 	}