Python subprocess.call() gives an error but same call in shell executes correctly

Question

I know there have been similar questions, but the solutions for those didn't help.

I'm writing a python script that calls a command line bash tool; I use subprocess.call() to execute it, like so:

def filter_data(var,rawdir,seq1,seq2,fn,pad=3,hdf='hdf'):
    """
    Inputs:
        var    -> variable you want filtered
        rawdir -> the directory where the raw MAS files are stored
        seq1   -> first number for which you want the operation executed
        seq2   -> last number for which you want the operation executed
        pad    -> zero-padding for MAS output files (usually either 3 or 6), default = 3
        hdf    -> whether the files are hdf or h5 (default is currently hdf, to match MAS)

    Outputs: the filtered set of files from seq1 to seq2 (inclusive)
    """
    
    # If the filtered-data directory doesn't exist, make it; either way, go there
    fildir='/'.join(rawdir.split('/')[0:-2])+'/hf'

    if os.path.exists(fildir) == False:
        os.mkdir(fildir)
    
    # Main filtering loop
    for i in range(seq1,seq2+1,1):
        os.chdir(rawdir)
        print("### Filtering data for sequence #"+str(i).zfill(pad))
        
        # Call the filter tool
        for i in range(len(fvars)):
            ivar=fvars[i]
            ovar=fildir+ivar[len(ivar)-(len(hdf)+len(var)+2+pad):]
            if os.path.isfile(ivar) == True:
                subprocess.call(['filter', '-n',str(fn), '-c', '-periodic z', ivar, ovar])     # this is where it's failing

When I run this, I get a syntax error from the filter tool, but if I paste the exact same command (filter -n $fn -c -periodic z /path/to/input/file/br020.hdf /path/to/output/file/br020.hdf), it executes properly and outputs the correct file. Anyone know what might be causing this error? Any help is appreciated!