LaTeX is expecting a number. If your command takes more than one argument, check to make sure the arguments are in the correct order. For example, if you are using a minipage environment, you might have omitted the mandatory argument which specifies the width of the minipage, or you may have the optional arguments the wrong way round. The placement specifier should come first, followed by the height.
If you are using
\setcounter remember that the second argument must be a number, so if you want
the value of a counter as the argument you must use
This error can be caused by, say,
This book is also available as A4 PDF or 12.8cm x 9.6cm PDF or paperback (ISBN 978-1-909440-00-5).