Non-integer axis mapping: Error: Base master not found

[hyvyys]

With the following axis entry in a DesignSpace file produced by FontLab 8:

  <axis minimum="100" maximum="1000" name="weight" default="767.716" tag="wght">
   <labelname xml:lang="en">Weight</labelname>
   <map input="100" output="100"/>
   <map input="200" output="127"/>
   <map input="300" output="164"/>
   <map input="400" output="210"/>
   <map input="500" output="271"/>
   <map input="600" output="348"/>
   <map input="700" output="447"/>
   <map input="767.716" output="533"/>
   <map input="800" output="574"/>
   <map input="900" output="737"/>
   <map input="1000" output="900"/>
  </axis>

with gftools 0.9.30, I'm getting the following error:

Generating fonts from Designspace failed: Base master not found.

Correcting the entry to the following (eliminating fractional mappings so that only integers are used), I get a correct build.

  <axis minimum="100" maximum="1000" name="weight" default="768" tag="wght">
   <labelname xml:lang="en">Weight</labelname>
   <map input="100" output="100"/>
   <map input="200" output="127"/>
   <map input="300" output="164"/>
   <map input="400" output="210"/>
   <map input="500" output="271"/>
   <map input="600" output="348"/>
   <map input="700" output="447"/>
   <map input="768" output="533"/>
   <map input="800" output="574"/>
   <map input="900" output="737"/>
   <map input="1000" output="900"/>
  </axis>
 </axes>

Can the mapping contain fractions by spec, or should I raise an issue with the FontLab team?