TI Inspire CX variable not defined in an if statement

I am attempting to write a modular inverse function based on this c++ version in my TI-Nspire CX.

So far I have the following code:

Define LibPub modinv(a,b)=
Func
 Local b0,t,q,x0,x1
 0→x0
 1→x1
 If b=1 Then
   Return 1
 EndIf
 While a>1
 q:=((a)/(b))
 t:=b
 b:=mod(a,b)
 a:=t
 t:=x0
 x0:=x1-q*x0
 x1:=t
 EndWhile
 If x1<0 Then
   x1:=x1+b0
 EndIf
 Return x1
 EndFunc

When I run the program

invmod(26,3)

I get this error:

Error: Variable is not defined

When I "view" the error it shows this line as being problematic (I think), as the cursor goes right to this line.

You didnt define b0, it has symbolic value. Since it is a local variable, you can not leave the scope, i.e. return x1 that contains symbolic b0 within.