Different node symbols for radiation pattern with d3.js

How to display nodes as different characters in a d3.js-forced library? I wanted to implement something similar to what I wrote below:

  var node = svg.selectAll(".node") 
      .data(graph.nodes) 
    .enter().append(function(d){return d.shape;}) 
      .attr("class", "node") 
      .attr("r", 5) 
      .style("fill", function(d) { return color(d.group); }) 
      .call(force.drag); 

Each node will have an encoded shape ("rect", "circle", etc.). However, I get an error message:

Uncaught TypeError: Object function (d){return "circle";} has no method 'indexOf' 

Another question I related to this is this: how would I switch between applying different attributes for each shape? Circles need the r attribute, but rectangles require height and width. Thank!