Particle and Filename problem

I have this code in a folder named particles :

class ParticleEngine(DirectObject):
	def __init__(self):
		#More standard initialization
		self.accept('escape', sys.exit)
		self.accept('a', self.loadParticleConfig , ['steam.ptf'])
		self.accept('b', self.loadParticleConfig , ['dust.ptf'])
		self.accept('c', self.loadParticleConfig , ['fountain.ptf'])
		self.accept('d', self.loadParticleConfig , ['smoke.ptf'])
		self.accept('e', self.loadParticleConfig , ['smokering.ptf'])
		self.accept('f', self.loadParticleConfig , ['fireish.ptf'])
		self.accept('g', self.loadParticleConfig , ['bluetrail1.ptf'])
		self.accept('h', self.loadParticleConfig , ['bluetrail2.ptf'])
		self.accept('escape', sys.exit)
		base.setBackgroundColor( 0, 0, 0 )

		self.t = loader.loadModel("models/teapot")
		self.p = ParticleEffect()

	def loadParticleConfig(self,file):
		#Start of the code from steam.ptf
		#Sets particles to birth relative to the teapot, but to render at toplevel
		self.p.setPos(2.000, 0.000, 3.000)

When i make an instance if this class from a file in the particles folder, it works, but when i make an instance importing it from the parent folder, it crashes with this error :
Assertion failed: okflag at line 121 of built/include/virtualFileSystem.I
Traceback (most recent call last):
File “./”, line 16, in
P = ParticleEngine()
File “/home/arikel/programmation/python/panda3d/morpg/client/particles/”, line 38, in init
File “/home/arikel/programmation/python/panda3d/morpg/client/particles/”, line 42, in loadParticleConfig
File “/usr/share/panda3d/direct/particles/”, line 201, in loadConfig
data = vfs.readFile(filename, 1)
AssertionError: okflag at line 121 of built/include/virtualFileSystem.I

Also, it looks like self.p.loadConfig(Filename(file)) is ok as look as file is something like “effect.ptf”, but not “particles/effect.ptf”. I found no solution to this problem on the forums…