Bug? (subgraph/cluster not showing up)

so for some reason in this example the subgraph is not showing up - even stranger:
commenting out the last 3 lines it does, but displays the label “grid” ie the name of the parent. so I was wondering is this intended ?

cheers
–a

graph grid
{
fontname=“Helvetica,Arial,sans-serif”
node [fontname=“Helvetica,Arial,sans-serif”]
edge [fontname=“Helvetica,Arial,sans-serif”]
layout=dot
label=“grid”
labelloc = “t”
ranksep=0.5;
nodesep=0.5;

edge [weight=1000  color=lightgrey]

subgraph cluster_0 {
B1 – B2
C1 – C2

}

A0 -- A1 -- A2 -- A3 
B0 -- B1 
B2 -- B3 

C0 -- C1 
C2 -- C3 
D0 -- D1 -- D2 -- D3 

rank=same {A0 -- B0 -- C0 -- D0} 
rank=same {A1 -- B1}   
rank=same {C1 -- D1} 
// comment out below to make the label "grid" appear for subgraph
rank=same {A2 -- B2} 
rank=same {	C2 -- D2} 
rank=same {A3 -- B3 -- C3 -- D3}

}