Using xacro to automate inertial calculations sequence/float error

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Running Ubuntu 16.04 with ROS-Kinetic I am trying to automatically calculate inertial values for a simulated robot. The purpose is to be able to change the dimensions and have the inertia automatically calculated. Here is my macro

  <xacro:macro name="cuboid_inertial" params="mass length width height">
    <inertial>
      <mass value="${mass}"/>
      <inertia 
                ixx="${0.83*mass*(${width}+${height})}" 
                ixy="0.0" 
                ixz="0.0" 
                iyy="${0.83*mass*(${length}+${height})}" 
                iyz="0.0" 
                izz="${0.83*mass*(${width}+${length})}" />
    </inertial>
</xacro:macro>

I have xacro properties as follows.

  <xacro:property name="base_link_length" value="0.50"/>
  <xacro:property name="base_link_width" value="0.30"/>
  <xacro:property name="base_link_height" value="0.35"/>
  <xacro:property name="base_link_length_sqrd" value="${base_link_length}*${base_link_length}"/>
  <xacro:property name="base_link_width_sqrd" value="${base_link_width}*${base_link_width}"/>
  <xacro:property name="base_link_height_sqrd" value="${base_link_height}*${base_link_height}"/>

When I try to run the macro it comes up with an error. My link tags...

  <link name="base_link">
    <visual>
      <geometry>
        <box size="${base_link_length} ${base_link_width} ${base_link_height}"/>
      </geometry>
      <material name="grey"/>
    </visual>
    <collision>
      <geometry>
        <box size="${base_link_length} ${base_link_width} ${base_link_height}"/>
      </geometry>
    </collision>
        <xacro:cuboid_inertial mass="{base_link_mass}" length="{base_link_length_sqrd}" width="{base_link_width_sqrd}" height="{base_link_height_sqrd}"/>
  </link>

The error is unexpected EOF while parsing (<string>, line 1) when evaluating expression '(.82)*${mass'