[Python] Sympy won't accept I as a symbol while differentiating

I made a simple program to calculate std for a given set of numbers and a function. While inputing some data today I realized that if one of the variables is called I it gives 0 as the derivative.

Here is the relevant part of the code:

for i in range(50):
des=input(f'Introduce variable: ')
if des == str(0):
break
variables.append(des)

valores=[]
incertidumbres=[]
for i in range(len(variables)):
valores.append(float(input(f'Dame el valor de la variable {variables}: ')))
incertidumbres.append(float(input(f'Dame el valor de la incertidumbre de {variables}: ')))
variables=sp.symbols(variables)

parciales=[]
ponderado=[]
for i in range(len(variables)):
parciales.append(sp.diff(funcion,variables))
print(f"La derivada de la función respecto a {variables} es {parciales}")
parciales=sp.lambdify(variables,parciales)
ponderado.append(parciales(*valores)**2*incertidumbres**2)


Sorry for the variables being in spanish.

If I input the ecuation a+b i get 1 and 1 as derivatives but i get 0 and 1 when I input I+a.

I was expecting the results to be consistent. Changing the name of the variable shouldn't affect the result, right?

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