This invention relates to a sleeve and a modular wind turbine blade comprising such a sleeve. The modular wind turbine blade comprises a first blade and a second blade section, wherein the two blade sections are joined together to form a joint interface having a number of adjoining end lines located in the outer surfaces. A sleeve is positioned over the joint interface and connected to both the first and second blade sections. The body of the sleeve extends over the adjoining end lines and protects them from environmental and external impacts. The sleeve further comprises a number of airflow modifying elements projecting from the outer surface of the sleeve. The airflow modifying elements may be stall fences.