Webwork compares functions by plugging in some points and seeing if they match. This error means that Webwork failed when trying to compare the student's function/equation to the correct function/equation. This occurs when the numbers it tries to plug in to the correct function are not in the domain. It can also occur due to computational limitations of WeBWorK's Perl-based arithmetic engine:

• The calculation involved numbers that were too big.
• The calculation involved imaginary numbers.
• The calculation involved non-integer powers of negative numbers.

To fix this, you can explicitly tell Webwork which points to plug in to the functions. Replace, e.g.,

fun_cmp("[correct answer]")

by

fun_cmp("[correct answer]", limits => [a,b] )

where [a,b] is the interval from which Webwork should pick test points (note that the default test interval for each interval is [-2,2]), or by

fun_cmp("[correct answer]", test_points=>[a,b,c,d,e] )

where a, b, c, d, and e are the specific points which Webwork should test. See http://webwork.maa.org/wiki/FormulaTestPoints for more details.