FindMinimum Error - cvec: Constrained optimization is only supported with scalar valued variables

Question

I want to solve a non-linear equation derived with a forward march finite difference technique (defined as "f" in the code). I am using FindMinimum, since I know an approximate initial value for each point.

FindMinimum returns the following error:

cvecv : "Constrained optimization is only supported with scalar valued variables"

How do I resolve this error? All values are scalars.

(* Variable definition *)
zeta = w*(Sqrt[scurr*scurr + (1 + 1/w - lcurr1)^2] - 1); 
psi = psist*(zeta^(-12) - 2/zeta^6); 
K = (A/Rv^2)*Exp[-psi]; 
fact = nu0*(1 + sigma) + 1/K; 
chi = 0.5*(fact - Sqrt[fact^2 - 4*nu0*nu0*sigma]); 
y[i] = (lcurr1 - l[i - 1])/(scurr - s[i - 1]); 
c[i] = (lcurr1 - 2*l[i - 1] + l[i - 2])/(scurr - s[i - 1])^2; 
curv = Sqrt[1 + y[i]^2]; 
omega1 = 2*Pi*scurr*curv*12*psist*(1/zeta^12 - 1/zeta^6)*(1 - lcurr1 + 
1/w)*w*(w/(zeta*(zeta + w)))*chi*(1 - (psi/K)*(1/(nu0*(1 + sigma) + 1/K))); 
xi1 = 2*Pi*kb*(-y[i]^2/curv^2 + 1 - (3*scurr*y[i]*c[i])/curv^4)*(-2/\[Beta] 
+ y[i]/(scurr*curv) + c[i]/curv^3) + 
2*Pi*scurr*y[i]*((chi*psi + 0.5*kb*(-2/\[Beta] + y[i]/(scurr*curv) + c[i]/curv^3)^2)/curv); 
pai1 = 2*Pi*kb*((y[i] + y[i]^3 + scurr*c[i])/curv^5 - (2*scurr)/(\[Beta] + \[Beta]*y[i]*y[i])); 

(* Objective function definition *)
f[lcurr1] = omega1 - (xi1 - xi[i - 1])/(s[i] - s[i - 1]) + (pai1 - 2*pai[i - 1] + pai[i - 2])/(s[i] - s[i - 1])^2; 
guess = l[i - 1]; 
sol = FindMinimum[{f[lcurr1], lcurr1 >= 0, lcurr1 <= 4},{lcurr1, guess}];