Replace empty values with values of other records in a dataframe

I have the following problem

I need to replace the empty cel values in dataframe with the values of the costs of CustomerNr of other records

df1 = pd.DataFrame([[1004,''], [1004, 'D'],[1005, 'C'], 
                   [1010,'A'], [1010,''],[1010,''],[1010,''],[1004, '']], columns=['CustomerNr','Costs'])

CustomerNr	Costs
1004
1004	D
1005	C
1010	A
1010
1010
1010
1004

Desired output:

CustomerNr	Costs
1004	D
1004	D
1005	C
1010	A
1010	A
1010	A
1010	A
1004	D