Using blkbuild with symbolic functions to find the transfer function of a block diagram

In the block diagram represented by the following code blocks 1 to 7 are defined as x, where x is a symbolic variable. When the code is executed I get a number of error messages although if n1 through n7 were numeric and not symbolic variables there would be no error. Can someone please explain how the blocks can be expressed as symbolic functions?

syms  x
n1 = x; d1 = 1;
n2 = x; d2 = 1;
n3 = x; d3 = 1;
n4 = x; d4 = 1;
n5 = x; d5 = 1;
n6 = x; d6 = 1;
n7 = x; d7 = 1; 
nblocks = 7; 
blkbuild
q = [1 0 0 0 0; 2 1 -5 0 0; 3 2 -6 0 0; 4 2 -6 3 -7; ...
    5 3 0 0 0; 6 3 0 0 0; 7 4 0 0 0]
input = 1; output = 4; 
[aa,bb,cc,dd] = connect(a,b,c,d,q,input,output)
[num,den] = ss2tf(aa,bb,cc,dd)
printsys(num,den)