Buffer duration in SC / Bela

Hello,
Here is a small change in the code of the example "Hello.scd" which does not work : It seems that .duration does not exist or is not understood.on bela

s.waitForBoot {
	// Your code goes here.
	b = Buffer.read(s, Platform.resourceDir +/+ "sounds/a11wlk01.wav");
b.duration;
//b.duration.postln does not work either
	{
		BufRd.ar(1, b, SinOsc.ar(0.08) * BufFrames.ir(b))
	}.play;
};

There after the error log:

PROTECTED CALL STACK:
	Meta_MethodError:new	0x1b48ae0
		arg this = DoesNotUnderstandError
		arg what = nil
		arg receiver = nil
	Meta_DoesNotUnderstandError:new	0x1b49ca0
		arg this = DoesNotUnderstandError
		arg receiver = nil
		arg selector = /
		arg args = [ nil ]
	Object:doesNotUnderstand	0x18c7e20
		arg this = nil
		arg selector = /
		arg args = nil
	a FunctionDef	0x24b11b0
		sourceCode = "{
	// Your code goes here.
	b = Buffer.read(s, Platform.resourceDir +/+ \"sounds/a11wlk01.wav\");
b.duration;
	{
		BufRd.ar(1, b, SinOsc.ar(0.08) * BufFrames.ir(b))
	}.play;
}"
	Routine:prStart	0x1fe8520
		arg this = a Routine
		arg inval = 7.473800079
CALL STACK:
	DoesNotUnderstandError:reportError
		arg this = 
	< closed FunctionDef >
		arg error = 
	Integer:forBy
		arg this = 0
		arg endval = 0
		arg stepval = 2
		arg function = 
		var i = 0
		var j = 0
	SequenceableCollection:pairsDo
		arg this = [*2]
		arg function = 
	Scheduler:seconds_
		arg this = 
		arg newSeconds = 13.802333348
	Meta_AppClock:tick
		arg this = 
		var saveClock = 
	Process:tick
		arg this = 
^^ The preceding error dump is for ERROR: Message '/' not understood.
RECEIVER: nil

Can someone help me corecting this ? ( I need the duration of the buffer fora Grain synth.)
thank you
JM