The lack of isolated X-ray pulsars with spin periods longer than 12 s raise. The question of wher. The population of evolved high-magnetic-field neutron stars has gone. Unlike canonical radiopulsars, X-ray pulsars are not subject to physical limits to the emission mechanism nor observational biases agains. The detection of sources with longer periods. Here we show that a highly resistive layer in the innermost part of the crust of neutron stars naturally limit. The spin period to a maximum value of about 10-20 s. This highly resistive layer is expected in the inner crust is amorphous and heterogeneous in nuclear charge, possibly owing to the existence of a nuclear 'pasta' phase. Our findings suggest tha. The maximum period of isolated X-ray pulsars may b. The first observational evidence for an amorphous inner crust, whose properties can be further constrained by future X-ray timing missions combined with more detailed models. © 2013 Macmillan Publishers Limited. All rights reserved.