DataFrame pandas where cell = empty, assign category not working, assigns it to everything

Let's say I have a table like this:

| Receipient account nr| Name      |Category|
| -------- ------------| --------- |--------|
| EE123123-------------|David------|gibberish|
| EE1231212------------|Mike-------|gibberish|
|----------------------|Maxima-----|gibberish|
|----------------------|Konsum-----|gibberish|
|----------------------|nikeaadidas|gibberish|

I'd like it to assign the Category on rows where "Receipient account nr" is not empty (contains IBAN number) to 'Transfer'

I use the following line to do so:

df['Category'] = np.where(df['Receipient account nr'] != '', 'Transfer', df['Category'])

However, when I run this line, it assigns all the categories to 'Transfer', so the table will look like this:

| Receipient account nr| Name      |Category|
| -------- ------------| --------- |--------|
| EE123123-------------|David------|Transfer|
| EE1231212------------|Mike-------|Transfer|
|----------------------|Maxima-----|Transfer|
|----------------------|Konsum-----|Transfer|
|----------------------|nikeaadidas|Transfer|

This is my preferred outcome:

| Receipient account nr| Name      |Category|
| -------- ------------| --------- |--------|
| EE123123-------------|David------|Transfer|
| EE1231212------------|Mike-------|Transfer|
|----------------------|Maxima-----|gibberish|
|----------------------|Konsum-----|gibberish|
|----------------------|nikeaadidas|gibberish|