Xacro Error: NoneType object has no attribute getitem

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I'm trying to convert my tortoisebot.urdf.xacro file to a tortoisebot.urdf file by running rosrun xacro xacro tortoisebot.urdf.xacro > tortoisebot.urdf When I run this command I get this error:

~/tortoisebot_ws/src/tortoisebot_description/robots$ rosrun xacro xacro tortoisebot.urdf.xacro > tortoisebot.urdf
Traceback (most recent call last):
  File "/opt/ros/indigo/lib/xacro/xacro", line 55, in <module>
    xacro.main()
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 696, in main
    eval_self_contained(doc)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 626, in eval_self_contained
    eval_all(doc.documentElement, macros, symbols)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 553, in eval_all
    eval_all(body, macros, scoped)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 609, in eval_all
    result = eval_text(at[1], symbols)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 483, in eval_text
    results.append(handle_expr(lex.next()[1][2:-1]))
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 470, in handle_expr
    return eval_expr(lex, symbols)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 444, in eval_expr
    result = eval_term(lex, symbols)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 418, in eval_term
    result = eval_factor(lex, symbols)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 402, in eval_factor
    result = eval_expr(lex, symbols)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/xacro/__init__.py", line 439, in eval_expr
    if lex.peek()[0] == lex.OP:
TypeError: 'NoneType' object has no attribute '__getitem__'

Here is my Xacro file:

<?xml version="1.0"?>

<robot name="tortoisebot" xmlns:xacro="http://www.ros.org/wiki/xacro" >
    <xacro:property name="pi" value="3.14159265" />
    <xacro:property name="body_radius" value="0.17" />
    <xacro:property name="body_height" value="0.2" />
    <xacro:property name="body_mass" value="2.0" />

    <xacro:property name="rear_caster_radius" value="0.01" />
    <xacro:property name="rear_caster_mass" value="0.1" />

    <xacro:property name="wheel_radius" value="0.036" />
    <xacro:property name="wheel_height" value="0.025" />
    <xacro:property name="wheel_mass" value="0.1" />

    <xacro:property name="right_wheel_color" value="${red}" />
    <xacro:property name="left_wheel_color" value="${white}" />

    <xacro:macro name="inertia_cylinder" params="mass radius height">
        <inertial>
            <mass value="${mass}"/>
            <inertia ixx="${(${mass} / 12)*((3 * ${radius} * ${radius}) + (${height} * ${height}))}"
                ixy="0.0" ixz="0.0" iyy="${(${mass} / 12)*((3 * ${radius} * ${radius}) + (${height} * ${height}))}"
                iyz="0.0" izz="${(${mass} * ${radius} * ${radius}) / 2}"/>
        </inertial>
    </xacro:macro>

    <xacro:macro name="inertia_sphere" params="mass radius">
        <inertial>
            <mass value="${mass}"/>
            <inertia ixx="${2 * ${mass} * ${radius} * ${radius} / 5}"
                ixy="0.0" ixz="0.0" iyy="${2 * ${mass} * ${radius} * ${radius} / 5}"
                iyz="0.0" izz="${2 * ${mass} * ${radius} * ${radius} / 5}"/>
        </inertial>
    </xacro:macro>

    <xacro:property name="white">
        <material name="white">
            <color rgba="1, 1, 1, 1"/>
        </material>
    </xacro:property>

    <xacro:property name="green">
        <material name="green">
            <color rgba="0.05 0.75 0.05 1"/>
        </material>
    </xacro:property>

    <xacro:property name="black">
        <material name="black">
            <color rgba="1, 1, 1, 1"/>
        </material>
    </xacro ...