How fplot legend works with four subplots of legendre functions?

Hi, I have this code:

clear all
clc
syms x

f1(x)=legendreP(1,x);

f2(x)=legendreP(2,x);

f3(x)=legendreP(3,x);

f4(x)=legendreP(4,x);

subplot(2,2,1), fplot(f1,[-1 1]), title('Polinomio de grado 1')

grid on

xlabel('x')

ylabel('f(x)')

subplot(2,2,2), fplot(f2,[-1 1]), title('Polinomio de grado 2')

grid on

xlabel('x')

ylabel('f(x)')

subplot(2,2,3), fplot(f3,[-1 1]), title('Polinomio de grado 3')

grid on

xlabel('x')

ylabel('f(x)')

subplot(2,2,4), fplot(f4,[-1 1]), title('Polinomio de grado 4')

grid on

xlabel('x')

ylabel('f(x)')

I want to understand how it works the legend in this case. I cannot figure out how it is.

The idea here is plotting 4 differents legendre functions with different titles and legends in each subplot.

Actually, each first time that I run this code, Matlab show me this errors:

Warning: Error updating FunctionLine.

   Second input must be a scalar or vector of unique symbolic variables.

Warning: Error updating FunctionLine.

   Second input must be a scalar or vector of unique symbolic variables.

Warning: Error updating FunctionLine.

   Second input must be a scalar or vector of unique symbolic variables.

Warning: Error updating FunctionLine.

   Second input must be a scalar or vector of unique symbolic variables.

And I can't too figure out why this is happening, because the second time I run it, I get the graphic.

Could you, please, help me?

Thank you

Have a nice day.

[Walter Roberson]

In R2017b I do not observe any warning.