(pdfx) can not coerce

Hi:
Can somebody help me with the following error message when using the
pdfx function? I am trying to plot the probability density functions of
precipitation rates (rain) with the following dimensions:

Variable: rain
Type: double
Total Size: 151301808 bytes
             18912726 values
Number of Dimensions: 2
Dimensions and sizes: [123] x [153762]
Coordinates:
Number Of Attributes: 1
   _FillValue : 9.969209968386869e+36

123 is the number of times and 153762 is the total number of grid cells
(unstructured mesh). Here is part of the script that computes the PDF:

;calculates the probability density distributions:
nbin = 50.
opt = True
opt@bin_min = 0.
opt@bin_max = 550.
printVarSummary(rain)
pdf_rain = pdfx(rain,nbin,opt)

fatal:Argument type mismatch on argument (1) of (pdfx) can not coerce
fatal:["Execute.c":8128]:Execute: Error occurred at or near line 75 in
file densityFunctions.ncl

I did not have any issue using a coarser mesh for which the number of
grid cells is 40962 instead of 153762, so that do not make sense to me.
The entire ncl script (densityFunctions.ncl) can be found in the
directory /glade/scratch/laura/nrcm.2013-11-06/run.x4_153762

Thanks,
Laura

-- 
!----------------------------------------------------
Laura D. Fowler
Mesoscale and Microscale Meteorology Division (MMM)
National Center for Atmospheric Research
P.O. Box 3000, Boulder CO 80307-3000
e-mail: laura@ucar.edu
phone : 303-497-1628
!----------------------------------------------------
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