According to sustainability transitions theories, innovation policies should create protective spaces (‘niches’) for promising new technologies. Moreover they should support a cumulative process of market formation and growth. Based on results from comparative case studies of two competing technological innovation systems for heavy transport (biogas and electrification), this paper argues that these recommendations are contradictory when technology alternatives with different degrees of maturity compete for the same niche. Should innovation policies open up the niche for the promising but immature alternative, or should they continue to support the technology that already has attained a niche position? If this contradiction remains unsolved, there is a risk for conflicts that block the progress of both alternatives. The paper suggests that there is a need for differentiated policies to resolve the contraction. In order to facilitate further development of both systems, the paper suggests that niche nurturing for immature systems needs to be combined with redeployment into new market segments for more mature systems.