Decorate invalid default error message (#415)

lib/types.js

@@ -2828,7 +2828,19 @@ function Field(schema, opts) {
     // allowed instead).
     // http://apache-avro.679487.n3.nabble.com/field-union-default-in-Java-td1175327.html
     var type = this.type;
-    var val = type._copy(value, {coerce: 2, wrap: 2});
+    var val;
+    try {
+      val = type._copy(value, {coerce: 2, wrap: 2});
+    } catch (err) {
+      var msg = f('incompatible field default %j (%s)', value, err.message);
+      if (Type.isType(type, 'union')) {
+        msg += f(
+          ', union defaults must match the first branch\'s type (%j)',
+          type.types[0]
+        );
+      }
+      throw new Error(msg);
+    }
     // The clone call above will throw an error if the default is invalid.
     if (isPrimitive(type.typeName) && type.typeName !== 'bytes') {
       // These are immutable.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "avsc",
-  "version": "5.7.6",
+  "version": "5.7.7",
   "description": "Avro for JavaScript",
   "homepage": "https://github.com/mtth/avsc",
   "keywords": [

test/test_types.js

@@ -1585,7 +1585,7 @@ suite('types', function () {
           name: 'Person',
           fields: [{name: 'name', type: ['null', 'string'], 'default': ''}]
         });
-      });
+      }, /incompatible.*first branch/);
     });
 
     test('record default', function () {