python pandas apply if statement and groupby only for one category

Piggy backing of this question python pandas flag if more than one unique row per value in column

I want to apply the following rule only to rows with Type X.

df['Test_flag'] = np.where(df.groupby('Category').Code.transform('nunique') > 1, 'T', '')

Dataframe df:

    Code      |  Type  | Category  |    Count
    code1          Y        A          89734
    code1          Y        A          239487
    code2          Z        B          298787
    code3          Z        B          87980
    code4          Y        C          098454
    code5          X        D          298787
    code6          X        D          87980

Expected result:

    Code      |  Type  | Category  |    Count  | Test Flag
    code1          Y        A          89734
    code1          Y        A          239487
    code2          Z        B          298787
    code3          Z        B          87980
    code4          Y        C          098454
    code5          X        D          298787       T
    code6          X        D          87980        T

I tried this

  df['Test_flag'] = np.where((df['Type'] == 'X') &df.groupby('Category').Code.transform('nunique') > 1, 'T', '')

and I get the following error:

ValueError: operands could not be broadcast together with shapes (1,2199) (7620,)