When using the Geodatabase to Coverage Conversion tool, if the geodatabase records contain NULL values represented by empty/blank cells (spaces), you will get an error message. The geodatabase feature class may contain null values in a numeric field, but there is no equivalent for this in ArcInfo coverages. Therefore, it is necessary to replace the null values with a numeric equivalent.
The same is true for converting shapefiles to coverages when the shapefile's attribute table contains null values.
It is suggested that you use 0 (zero) as the numeric substitute, and therefore replace all of the null values in the geodatabase, or shapefile, with zeros; assuming this will not cause data interpretation problems.
If the null values need to be distinguished from zero values, then change these to a special identifier value within the geodatabase such as -99. Then these will not be changed to 0 values when converted to a coverage or info table.