Incorrect value of object if using type casting

Run code:

var sqlError:SQLError = new SQLError("test", "", "", 7);

trace("sqlError.errorID: " + sqlError.errorID);

trace("Error(sqlError).errorID: " + Error(sqlError).errorID);

trace("(sqlError as Error).errorID: " + (sqlError as Error).errorID);

Actual Result:

[trace] sqlError.errorID: 7

[trace] Error(sqlError).errorID: 0

[trace] (sqlError as Error).errorID: 7

Expected Result:

[trace] sqlError.errorID: 7

[trace] Error(sqlError).errorID: 7

[trace] (sqlError as Error).errorID: 7

Is it a runtime bug or "feature"?