I have the following data frames:

```
print(df_a)
     mukey  DI  PI
0   100000  35  14
1  1000005  44  14
2  1000006  44  14
3  1000007  43  13
4  1000008  43  13

print(df_b)
    mukey  niccdcd
0  190236        4
1  190237        6
2  190238        7
3  190239        4
4  190240        7

```

When I try to join these data frames:

```
join_df = df_a.join(df_b, on='mukey', how='left')

```

I get the error:

```
*** ValueError: columns overlap but no suffix specified: Index([u'mukey'], dtype='object')

```

Why is this so? The data frames do have common `'mukey'` values.