The Zero-impression behavior of each field is listed in the
report documentation. If a field does not support Zero-Impressions, this value is false and will raise an error if it is included in the report definition and the
includeZeroImpressions header is set to true. If a field does support zero impressions, then the inclusion of zero impressions depends on the value of the includeZeroImpressions header. I'll file a request to have the docs updated so this behavior is more clear.
Shwetha, AdWords API Team.