There is provided a data processing apparatus comprising: processing circuitry to speculatively execute an instruction referencing a virtual address. Lookup circuitry receives the virtual address from the processing circuitry. The lookup circuitry comprises storage circuitry to store at least one virtual address and page walking circuitry to perform a page walk on further storage circuitry, in dependence on the virtual address being unlisted by the storage circuitry, to determine whether a correspondence between a physical address and the virtual address exists. The lookup circuitry signals an error when the correspondence cannot be found and, in response to the error being signaled, the storage circuitry stores an entry comprising the virtual address.