diff --git a/asyncpg/exceptions/_base.py b/asyncpg/exceptions/_base.py
index c2d1e7ef..89ac6f99 100644
--- a/asyncpg/exceptions/_base.py
+++ b/asyncpg/exceptions/_base.py
@@ -65,7 +65,7 @@ def get_message_class_for_sqlstate(mcls, code):
 
 class PostgresMessage(metaclass=PostgresMessageMeta):
     def __str__(self):
-        msg = self.message
+        msg = self.args[0]
         if self.detail:
             msg += '\nDETAIL:  {}'.format(self.detail)
         if self.hint:
@@ -87,13 +87,13 @@ def new(cls, fields, query=None):
             if field:
                 mapped[field] = v
 
-        e = exccls(mapped.get('message'))
+        e = exccls(mapped.get('message', ''))
         e.__dict__.update(mapped)
 
         return e
 
 
-class PostgresError(Exception, PostgresMessage):
+class PostgresError(PostgresMessage, Exception):
     """Base class for all Postgres errors."""
 
 
diff --git a/tests/test_exceptions.py b/tests/test_exceptions.py
index d904f6b0..5149a65c 100644
--- a/tests/test_exceptions.py
+++ b/tests/test_exceptions.py
@@ -23,12 +23,28 @@ def test_exceptions_exported(self):
         self.assertIsNone(asyncpg.PostgresError.schema_name)
 
     async def test_exceptions_unpacking(self):
-        with self.assertRaises(asyncpg.UndefinedTableError):
-            try:
-                await self.con.execute('SELECT * FROM _nonexistent_')
-            except asyncpg.UndefinedTableError as e:
-                self.assertEqual(e.sqlstate, '42P01')
-                self.assertEqual(e.position, '15')
-                self.assertEqual(e.query, 'SELECT * FROM _nonexistent_')
-                self.assertIsNotNone(e.severity)
-                raise
+        try:
+            await self.con.execute('SELECT * FROM _nonexistent_')
+        except asyncpg.UndefinedTableError as e:
+            self.assertEqual(e.sqlstate, '42P01')
+            self.assertEqual(e.position, '15')
+            self.assertEqual(e.query, 'SELECT * FROM _nonexistent_')
+            self.assertIsNotNone(e.severity)
+        else:
+            self.fail('UndefinedTableError not raised')
+
+    async def test_exceptions_str(self):
+        try:
+            await self.con.execute('''
+                 CREATE FUNCTION foo() RETURNS bool AS $$ $$ LANGUAGE SQL;
+            ''')
+        except asyncpg.InvalidFunctionDefinitionError as e:
+            self.assertEqual(
+                e.detail,
+                "Function's final statement must be SELECT or "
+                "INSERT/UPDATE/DELETE RETURNING.")
+            self.assertIn(
+                'DETAIL:  Function', str(e)
+            )
+        else:
+            self.fail('InvalidFunctionDefinitionError not raised')