ValueError: Length mismatch: Expected axis has X elements, new values have Y elements

Question

I try to fill missing value with the most appeared one in its group . Code :

f = lambda x: x.mode().iat[0] if x.notna().any() else np.nan
s = df.groupby('VehicleType')['FuelType'].transform(f)
df['FuelType']=df['FuelType'].fillna(s)

Error: ValueError: Length mismatch: Expected axis has 316879 elements, new values have 354369 elements

Data sample :

Possible solutions: I think that maybe the VehicleType data has missing values, therefore it gives an error .Because when I use another column that has no missing values, it works. But I have to use VehicleType for this task .