fix nullable enums

Dapper/SqlMapper.cs

@@ -719,7 +719,15 @@ private static object GetStructDeserializer<T>(IDataReader reader)
                     il.Emit(OpCodes.Isinst, typeof(DBNull)); // stack is now [target][target][value-as-object][DBNull or null]
                     il.Emit(OpCodes.Brtrue_S, isDbNullLabel); // stack is now [target][target][value-as-object]
 
-                    il.Emit(OpCodes.Unbox_Any, item.Info.Type); // stack is now [target][target][typed-value]
+                    // unbox nullable enums as the primitive, i.e. byte etc
+                    var nullUnderlyingType = Nullable.GetUnderlyingType(item.Info.Type);
+                    var unboxType = nullUnderlyingType != null && nullUnderlyingType.IsEnum ? nullUnderlyingType : item.Info.Type;
+                    il.Emit(OpCodes.Unbox_Any, unboxType); // stack is now [target][target][typed-value]
+                    if (nullUnderlyingType != null && nullUnderlyingType.IsEnum)
+                    {
+                        il.Emit(OpCodes.Newobj, item.Info.Type.GetConstructor(new[] { nullUnderlyingType }));
+                    }
+
                     il.Emit(OpCodes.Callvirt, item.Info.Setter); // stack is now [target]
                     il.Emit(OpCodes.Br_S, finishLabel); // stack is now [target]