Error in Lordif: slope is missing or negative

Hi all

I am not an experienced user of R.
I am trying to perform DIF analysis using Lordif and I get the follow error:

GroupDIF <- lordif(resp.data=Resp, group=Group, criterion="R2", pseudo.R2="McFadden", R2.change=0.02)

Iteration: 500, Log-Lik: -137340.437, Max-Change: 0.00119
EM cycles terminated after 500 iterations.
 (mirt) | Iteration: 1, 14 items flagged for DIF (22,25,34,60,64,71,73,75,81,82,83,86,94,95)
Iteration: 500, Log-Lik: -131933.514, Max-Change: 0.00033
EM cycles terminated after 500 iterations.
ERROR: The following items had negative slope parameters.
87,89,91,93,95,99ERROR: The following items had negative slope parameters (87,89,91,93,95,99).

Error in probgrm(theta, DISC[i], CB[i, ]) : slope is missing or negative

Does anlyone knows what the poblem might be? Any help is appreciated!

Best regards
Caroline