Message "One or more master pages do not exist"

[Neubauer Martha]

Hi all, 
 
Windows XP SP 2 
FrameMaker 7.2 p158 
 
I've just started a new template with five Master pages: 
First 
Right 
Left 
LandscapeRight 
LandscapeLeft 
 
"First" ist mapped to the paragraph format "ChapterTitle" on the
Reference pages: 
UnstructMasterPageMaps
Book Update (Yes or No): Yes
Paragraph Tag;Name Right-Handed Master Page (or Single-Sided Master
Page);Left-Handed Master Page Range Indicator (Single, Span pages, Until
changed);Comments
ChapterTitle;First;First;Single  
 
I can apply any of the Master pages by using Format > Page Layout >
Master Page Usage. 
 
When I select Format > Page Layout > Apply Master Pages, I get the
following message: 
One or more master pages do not exist. Aborting "Apply Master Pages..."!

Does anyone know why I'm getting this message? 
 
 
Martha