Problems with the Frequency Xlating FIR Filter

[Michelle]

hello,

I'm using a FIR filter and I have the error : "Param - Taps(taps): 
Expression None is invalid for type'complex_vector'."

my taps is : firdes.low_pass ( 1, samp_rate, frec_carrier, 
25000,firdes.WIN_HAMMING )

samp_rate and  frec_carrier are both variables in my graph.

I don't understand why I have an error. Please could you tell me what I 
am doing wrong?


Thank you.