Symptoms

ExpressionEvaluator_C: @Evaluate -- failed to evaluate expression `expr *0.3048' -- missing operand at _@_in expression "_@_*0.3048"

Cause

The error "missing operand" in the ExpressionEvaluator is not due to syntax but a feature which has a null attribute value (or missing the attribute altogether) which is expected by the ExpressionEvaluator.

Resolution

Use a Tester beforehand to check where &attribute != ""