Sigma serial numbers start with SI, for SIgma. Not a 1 or an L but an i. Please make sure you are using SI when registering.
For example SI0987.
In addition, owners of Sigma with a serial number in the format SI2xxx may experience an Invalid ID number error when registering.
In such a case, please insert a 0 (zero) between the SI and 2xxx.
For example a serial of SI2000 becomes SI02000.