Query function to fetch the length of HashMap works inefficiently when the key is of type Vec<u8>

[harsh-supra]

I have created a contract which requires to query the length of a HashMap, So when the Key of this particular HashMap I use as Vec<u8> , the length is always returned as 0, whereas If I use String as the Key type, the function works perfectly fine. Am I missing something in here? Sharing the Query function snippet below for reference:

pub fn get_no_of_root_hashes(deps: Deps) -> Result<usize, ContractError> {
        let state = STATE.load(deps.storage)?;
        println!("Query: {:?}", state.root_hashes.len());
        Ok(state.root_hashes.len())
    }

#[derive(Serialize, Deserialize, Clone, Debug, PartialEq, Eq, JsonSchema)]
pub struct State {
    pub owner: cosmwasm_std::Addr,
    pub com_to_pub_key: HashMap<u64, Vec<u8>>,
    pub root_hashes: HashMap<Vec<u8>, u64>,
}

[chipshort]

What does the json look like that is saved inside STATE and how do you save it there?
When I try serializing a State struct, it correctly errors with "JSON object key is required to be a string type."