Qiao et al. recently presented a novel approach to building certificateless aggregate signature (CLAS) schemes that guarantee the authenticity of clinical record in healthcare wireless medical sensor networks (HWMSNs). They start by developing a certificateless signature technique, from which they propose a CLAS technique by incorporating an aggregational and a verification algorithm. We argue in this paper that the Type I attacker who is capable to access secret keys of medical sensor nodes can forge valid signatures, making their CLAS scheme insecure. That is, their CLAS scheme does not truly attain the unforgeability. Ultimately, we apply our cryptanalysis to the real-world use case. That is, the adversary can cast a real attack against their CLAS technique using our attack method to forge signatures in the practical application of HWMSNs. Thus, Qiao et al.’s CLAS technique is completely breakable.