exception when reading an IGES file after having reading a STEP file

use the following Draw Script :
##
puts "test "
puts " write STEP and IGES file "
box b 12 34 56
stepwrite 1 b /tmp/box.step
brepiges b /tmp/box.igs
##
puts " Read STEP file "
dall
stepread /tmp/box.step a *
dall
puts " Read IGES file "
igesbrep /tmp/box.igs b *

the following exception is occured :
An exception was caught 0xfdbb63f8 : Standard_NullObject: Attempt to access to
null object
** Exception ** 0xfdbb63f8 : Standard_NullObject: Attempt to access to null
object

Remark :

we have not this exception if we made :
first read the IGES file
second read the STEP file