# Error computing kernel of matrix over symbolic ring

I am trying to compute the kernel of the Laplacian of the hypercube graph where all nonzero entries are symbolic variables. The below code works for N=2

```
N = 3 #dimension of hypercube graph
G = graphs.CubeGraph(N).to_directed()
#Assign edge labels as symbolic variables
for u,v,l in G.edges(sort=False):
this_label = 'e' + str(u) + 'e' + str(v)
G.set_edge_label(u, v, this_label)
var(this_label)
#relabel vertices to facilitate conversion to matrix
G.relabel()
#Create edge dictionary to facilitate conversion to matrix
edge_dict = {(u,v):k for u,v,k in G.edges()}
#Construct Laplacian matrix
temp = matrix(SR, G.order(), G.order(), edge_dict)
L = temp - diagonal_matrix(sum(temp))
#compute Laplacian kernel
ker = transpose(L).kernel();
```

but gives the below error for N=3 at the last line:

```
TypeError: ECL says: THROW: The catch RAT-ERR is undefined.
During handling of the above exception, another exception occurred:
...
TypeError: Multiplying row by Symbolic Ring element cannot be done over Symbolic Ring, use change_ring or with_added_multiple_of_row instead.
```

Any ideas as to the cause of the error? Can SAGE handle computing kernels of symbolic matrices of this size?

Thanks!