I encountered it while executing from object_detection.utils import label_map_util in jupyter notebook. It is actually the tensorflow object detection tutorial
After trying many different solutions (i'm working on a Mac) the one that worked for me is to reinstall protobuf using:
PROTOC_ZIP=protoc-3.7.1-osx-x86_64.zip
curl -OL https://github.com/protocolbuffers/protobuf/releases/download/v3.7.1/$PROTOC_ZIP
sudo unzip -o $PROTOC_ZIP -d /usr/local bin/protoc
sudo unzip -o $PROTOC_ZIP -d /usr/local 'include/*'
rm -f $PROTOC_ZIP
As is highlighted in this article