COCO evaluation - Negative values on AP and AR

I am trying to evaluate one of the models using COCO dataset metrics. For some of the precision metrics I am getting results that I can comprehend. For example:
Average Precision (AP) @[ IoU=0.50:0.95 | area= small | maxDets=100 ] = 0.546
Average Recall (AR) @[ IoU=0.50:0.95 | area=small | maxDets=100 ] = 0.62

But for few of them I am getting -1 as the result. For example,
Average Precision (AP) @[ IoU=0.50:0.95 | area=medium | maxDets=100 ] = -1
Average Precision (AP) @[ IoU=0.50:0.95 | area=large | maxDets=100 ] = -1
Average Recall (AR) @[ IoU=0.50:0.95 | area=medium | maxDets=100 ] = -1
Average Recall (AR) @[ IoU=0.50:0.95 | area=large | maxDets=100 ] = -1

Not sure how to comprehend -1 for AP or AR.