A study of the in vitro nanoparticle-templated assembly of a mutant of cowpea chlorotic mottle virus lacking most of the N-terminal domain (residues 4-37), NDelta34, is presented. Mutant empty proteins assemble into empty capsids with a much broader distribution of sizes than the wild-type virus. This increased flexibility in the assembly outcomes is known to be detrimental for the assembly process in the presence of molecular polyanions. However, when rigid polyanionic cores are used, such as nanoparticles, the assembly process is restored and virus-like particles form. Moreover, the breadth of the nanoparticle-templated capsid size distribution becomes comparable with the wild-type virus size distribution.