Pandas groupby with delimiter join

I tried to use groupby to group rows with multiple values.

col val
A  Cat
A  Tiger
B  Ball
B  Bat

import pandas as pd
df = pd.read_csv("Inputfile.txt", sep='\t')
group = df.groupby(['col'])['val'].sum()

I got

A CatTiger
B BallBat

I want to introduce a delimiter, so that my output looks like

A Cat-Tiger
B Ball-Bat

I tried,

group = df.groupby(['col'])['val'].sum().apply(lambda x: '-'.join(x))

this yielded,

A C-a-t-T-i-g-e-r
B B-a-l-l-B-a-t

What is the issue here ?

Thanks,