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I'm trying to make a simple interactive for my students to practice differentiating polynomials. For some reason, when an answer is entered, Sage tries to convert it (a polynomial) to an instance of sage.structure.parent.Parent and chokes. The same code run outside of an interactive works fine. Why is Sage trying to do this conversion and how do I get it to stop?

#Create polynomial generator
poly.<x>=PolynomialRing(QQ)

#Problems correct (in a row and total)
streak = 0
tot = 0
first = True

@interact
def polyDiff(ans=input_box(label="f'(x)=", type=sage.rings.polynomial.polynomial_rational_flint.Polynomial_rational_flint)):
    #Load persistent variables
    global streak
    global tot
    global first

    global poly

    #Set up and display problem on first run
    if (first == True):
         rand_poly=poly.random_element(2)
         first = False
    html("f(x)=")+html(rand_poly)

    #Define correct answer
    corr = diff(rand_poly, x)

    #Check student's answer
    if (ans is not None):
        if (corr == ans):
            print "Correct!"
            tot += 1
            streak += 1
            #Generate new problem
            rand_poly = poly.random_element(5)
        else:
            html("Try again")
            streak = 0
    else:
        print "Enter your answer"