AttributeError: 'float' object has no attribute 'lower'

def clean_strings(strg):
    strg = strg.lower() #lower
    return strg

df_sample['clean_content'] = df_sample['content'].apply(clean_strings)

df_sample['clean_content']= df_sample['content'].map(clean_strings)

df['content'] = df['content'].astype(str)

def clean_strings(strg):
    strg= strg.lower() #lower
    return strg

def clean_string(row):
    try:
        return row.lower()
    except AttributeError:
        return row

df_sample['clean_content'] = df_sample['content'].apply(clean_string)

df_sample['content'] = df_sample['content'].astype(str)

df_sample['content'] = df_sample['content'].astype(str).str.lower()

df_sample['content'] = df_sample['content'].str.lower()  # assuming `content` is of `object` type