Error: Negative sweep data in curve-fit file

Hello,

I encounter an error when using Optimizer tool for curve fitting. The error says that I have negative values in my reference file but I don't understand how it is a problem.

I am doing a parameter sweep from -10 to +40 with a step of 1.

Here is the content of my reference file :

temperature    V(Vadj2)
-10    2.45
-9    2.437
-8    2.424
-7    2.411
-6    2.398
-5    2.385
-4    2.372
-3    2.359
-2    2.346
-1    2.333
0    2.32
1    2.307
2    2.294
3    2.281
4    2.268
5    2.255
6    2.242
7    2.229
8    2.216
9    2.203
10    2.19
11    2.177
12    2.164
13    2.151
14    2.138
15    2.125
16    2.112
17    2.099
18    2.086
19    2.073
20    2.06
21    2.047
22    2.034
23    2.021
24    2.008
25    1.995
26    1.982
27    1.969
28    1.956
29    1.943
30    1.93
31    1.917
32    1.904
33    1.891
34    1.878
35    1.865
36    1.852
37    1.839
38    1.826
39    1.813
40    1.8

Thank you in advance for any help.