The implementation of Indonesia’s national health insurance programme in 2014 highlights the “missing middle” problem in which non-poor workers in the informal sector remain uncovered from the health care due to self-enrolment. This study aims at examining why informal workers are reluctant to join the national health insurance even though the benefits of the programme are very generous. Observing 400 households working in the informal sector and applying the Triple Bounded Dichotomies Choice Contingent Valuation Method to observe the Willingness to Pay, this study found that 70 per cent of respondents were willing to pay a premium that is lower than the current rate. Yet, only 18.7 per cent of households had registered for national health insurance. Our econometric estimations provide evidence that the availability of hospitals, insurance literacy, experiences of being an inpatient or outpatient, the number of family members, the sex of the head of the household, access to the Internet, and household income are highly correlated to the likelihood of workers in the informal sector joining the national health insurance programme. In contrast to other studies, we found that the insurance premium was not the primary impediment. Rather, the two main obstacles were: availability of health services; and a lack of insurance literacy. Hence, this study calls for increased investment on healthcare facilitates as well as campaigns to educate the public about the importance of health insurance.