Can't susbsitute a variable that is defined

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I have solved a system of ODEs using dsolve. They create a set of symbolic variables inside the solutions when integrating as expected. When I check to see the name of these integrationg constants one of these is called "C1". Now, I want to substitue using subs that integration variable for a number.
symvar(Vb_sol) %Check the name of the constants inside my ODE solution
V_sub = subs(Vb_sol,C1,1) %Replace C1 (integration constant) by 1
However, when I try do do the substitution I get the "Undefined function or variable 'C1'" error.
I have attached the variable "Vb_sol", which is a function of C1, and an example script trying to do the substituion.
Thank you

Accepted Answer

Paul on 17 Feb 2023
Edited: Paul on 17 Feb 2023
syms C1
before calling subs.
syms y(t) a
eqn = diff(y,t) == a*y;
S = dsolve(eqn)
S = 
syms C1
ans = 

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