diff -ur jna-3.0.4-svn630/src/com/sun/jna/Structure.java jna-3.0.4-svn630.orig/src/com/sun/jna/Structure.java
--- jna-3.0.4-svn630/src/com/sun/jna/Structure.java	2008-07-31 13:44:21.000000000 -0400
+++ jna-3.0.4-svn630.orig/src/com/sun/jna/Structure.java	2008-09-05 21:47:51.000000000 -0400
@@ -126,8 +126,16 @@
     }
 
     protected Structure(int size, int alignment) {
+	this(size, alignment, null);
+    }
+
+    protected Structure(TypeMapper mapper) {
+	this(CALCULATE_SIZE, ALIGN_DEFAULT, mapper);
+    }
+
+    protected Structure(int size, int alignment, TypeMapper mapper) {
         setAlignType(alignment);
-        setTypeMapper(null);
+        setTypeMapper(mapper);
         allocateMemory(size);
     }
 
@@ -974,7 +982,8 @@
         }
         catch(IllegalArgumentException e) {
             throw new IllegalArgumentException("The type \"" + type.getName()
-                                               + "\" is not supported as a structure field");
+                                               + "\" is not supported as a structure field: "
+					       + e.getMessage());
         }
     }
 
diff -ur jna-3.0.4-svn630/src/com/sun/jna/Union.java jna-3.0.4-svn630.orig/src/com/sun/jna/Union.java
--- jna-3.0.4-svn630/src/com/sun/jna/Union.java	2008-07-31 13:44:21.000000000 -0400
+++ jna-3.0.4-svn630.orig/src/com/sun/jna/Union.java	2008-09-05 21:47:18.000000000 -0400
@@ -39,6 +39,11 @@
     protected Union(int size, int alignType) {
         super(size, alignType);
     }
+
+    protected Union(TypeMapper mapper) {
+	super(mapper);
+    }
+
     /** Indicates which field will be used to write to native memory. 
      * @throws IllegalArgumentException if the type does not correspond to 
      * any declared union field.