InverseMillsRatio = NULL, running selection() model

Question

I have been struggling with this problem the last couple days, whenever I run my selection model, the IMR obtained is equal to NULL. Does anyone know how I can adjust my code to obtain a value? Thank you very much.

Variables used:

EarlyFunding - binary, whether a start-up received early funding or not

NumberofFounder - numeric, how many founders the start-up has

PriceUSD - numeric, start-up acquisition price

CapitalAllocated - numeric, how much capital was allocated to the start-up

DaysActive - numeric, days active before acquisition

MicroSize, SmallSize, LargeSize, XLargeSize - dummy variables, either 1 or 0

NumberofFunding - numeric, how many funding rounds the start-up underwent

My code in:

heckit <- selection(EarlyFunding ~ NumberofFounder,
                    log(PriceUSD) ~ log(CapitalAllocated) + DaysActive + MicroSize + SmallSize + LargeSize + XLargeSize + NumberofFunding + EarlyFunding)
summary(heckit)$Mills

Console Output:

Tobit treatment model (switching regression model)
Maximum Likelihood estimation
Newton-Raphson maximisation, 4 iterations
Return code 8: successive function values within relative tolerance limit (reltol)
Log-Likelihood: -1805.498 
816 observations: 470 non-participants (selection 0) and 346 participants (selection 1)

13 free parameters (df = 803)
Probit selection equation:
                Estimate Std. Error t value      Pr(>|t|)    
(Intercept)     -0.56529    0.09326  -6.062 0.00000000207 ***
NumberofFounder  0.17500    0.03455   5.066 0.00000050558 ***
Outcome equation:
                          Estimate   Std. Error t value             Pr(>|t|)    
(Intercept)           19.178626134  0.212762493  90.141 < 0.0000000000000002 ***
log(CapitalAllocated) -0.687235433  0.026411267 -26.021 < 0.0000000000000002 ***
DaysActive             0.000006998  0.000025479   0.275             0.783647    
MicroSize             -2.242781147  0.206837357 -10.843 < 0.0000000000000002 ***
SmallSize             -1.131765568  0.109541607 -10.332 < 0.0000000000000002 ***
LargeSize              0.498545219  0.136097816   3.663             0.000265 ***
XLargeSize             1.024228327  0.109721965   9.335 < 0.0000000000000002 ***
NumberofFunding        0.226500314  0.017561332  12.898 < 0.0000000000000002 ***
EarlyFunding           1.648318372  0.287864038   5.726         0.0000000145 ***
   Error terms:
      Estimate Std. Error t value             Pr(>|t|)    
sigma  1.36322    0.08403  16.224 < 0.0000000000000002 ***
rho   -0.70534    0.08625  -8.178  0.00000000000000112 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1
--------------------------------------------
> summary(heckit)$Mills
NULL
>