You are overwriting "m" on each iteration of the "for i" loop.
Should we assume that "x" is defined as a symbol?
Please check that "i" is the variable name being used in your loop. Using "i" as a variable name is not a good idea as it clashes with the use of "i" meaning the square root of negative 1.
Note that the "x" of @(x) will not be the same "x" as in "dy". When a dummy argument name is used in an anonymous function, the dummy argument value will only be substituted for variables with that name that occur literally in the anonymous function body. You could, for example, use
g = @(x) subs(dy, 'x', x)
provided that dy is symbolic.
