diff --git a/src/factory/PVUnion.cpp b/src/factory/PVUnion.cpp
index 06e7be3..804b7fb 100644
--- a/src/factory/PVUnion.cpp
+++ b/src/factory/PVUnion.cpp
@@ -25,7 +25,7 @@ using std::string;
 namespace epics { namespace pvData {
 	
 #define PVUNION_UNDEFINED_INDEX -1
-int32 PVUnion::UNDEFINED_INDEX = PVUNION_UNDEFINED_INDEX;
+const int32 PVUnion::UNDEFINED_INDEX = PVUNION_UNDEFINED_INDEX;
 
 PVDataCreatePtr PVUnion::pvDataCreate(getPVDataCreate());
 
diff --git a/src/pv/pvData.h b/src/pv/pvData.h
index 297b7f7..3d83573 100644
--- a/src/pv/pvData.h
+++ b/src/pv/pvData.h
@@ -932,7 +932,7 @@ public:
 	 * Default value upon PVUnion construction. Can be set by the user.
 	 * Corresponds to @c null value.
 	 */
-	static int32 UNDEFINED_INDEX;
+    static const int32 UNDEFINED_INDEX;
 
     /**
      * Get the introspection interface