Interpreting their experimental data in terms of an approximate solution of the Lawrence - Doniach - Clem (LDC) model in the continuum limit (describing an isolated interlayer Josephson vortex), Moler et al (1998 Science 279 1193) have estimated the penetration depth in the direction normal to the layers of the compound (Tl-2201) to have the value . They thus concluded that only of the superconducting condensation energy in Tl-2201 would be due to the interlayer-tunnelling mechanism. We have studied the LDC model and found that it has no physical solution. Therefore in Tl-2201 is in need of re-examination.