This case study was carried out with a student who was 9 years old and in the fourth grade when the study began. She was selected after the administration of an exploratory questionnaire to 66 pupils in primary school. The main features of the case were the absence of a common referent and a corresponding unit to the free generation of references related to the addition of fractions. The girl was interviewed twice with the same instrument. The interviews consisted of 10 tasks. The link generated by this student between different classes of objects (referents) through a sum affected only fraction references (not natural number references). Likewise, the child was unable to establish a unit when facing the requirement to construct the additive situation by her own means while she could assign sense, recognize and select an adequate unit in simple tasks, and reconstruct whole processes. Contains 15 references. (Author)