Cannot export graphic or equation contents. Check the element mapping and description.

Hi,

I am using DITA 1.3 Concept to create a topic. There is a equation in it.

I initially insert the equation in element <equation-block> using the FrameMaker equation pod. FrameMaker create an element <EQUATION> which is a invalid element. So I add an element definition <equation> in the EDD and allow it to be a child element of <equation-block>.

The <equation> definition is:

Element (Equation): equation
Attribute list
Name: id        Unique ID       Optional
Name: class     String  Optional

I also add a read/write rule for <equation>, like this:

element "image" {
  is fm equation element "equation";
}

When I save the concept topic with a equation, there always is a error message shown on the console:

XML Write Messages…
Cannot export graphic or equation contents. Check the element mapping and description.

I have modified the read/write rules for several times, but this error message is always there.

If I colse the topic and reoped it again, the equation disappears. An error message is shown on the console too:

XML Read Messages (Document Instance)
Cannot find the file () containing the imported graphic.

How to solve this?