I can't call a FastAPI POST route using Python's "requests" module, but I'm able to call the same route via cURL command line

Question

Below you can find (the relevant excerpt of) my "server-side" FastAPI code.

from fastapi import FastAPI, UploadFile, HTTPException
from fastapi.responses import FileResponse, HTMLResponse, StreamingResponse
from pydantic import BaseModel

qh = os.getenv('QDRANTHOST','qdrant')
qc = os.getenv('QDRANTCOLL','documents')

class QdrantQuery(BaseModel):
    query: str
    host: str | None = qh
    collection: str | None = qc

def execute(question: str, host: str, collection: str):
    index = qdrant_index(qdranthost=host,collection=collection)
    query_engine = index.as_query_engine(similarity_top_k=int(similarity),response_mode=responsemode)
    return query_engine.query(question)

@app.post("/query")
async def query(input: QdrantQuery ):
    question = input.query
    host = input.host
    collection = input.collection
    result = execute(question,host,collection)
    return result

When I call this code from a BASH shell, with this command line:

curl -H 'content-type: application/json' -d '{"query": "Chi è Blair Clarimonde?","collection": "documents"}' http://localhost:9999/query

It works perfectly. However, when calling from within a Streamlit frontend, through requests module... I keep getting 422 error codes

queryurl = f"http://dctvllm01.upgrade.lan:9999/query"
headers={"Content-type": "application/json"}
a = {"question": "Chi è Blair Clarimonde?","collection": "documents"}
b = json.dumps(a)
r = requests.post(url=queryurl,data=b, headers=headers)
print(r.text)

Error detail:

{"detail":[{"type":"missing","loc":["body","query"],"msg":"Field required","input":{"question":"Chi è Blair Clarimonde?","collection":"documents"},"url":"https://errors.pydantic.dev/2.6/v/missing"}]}

Could someone point out the fault in my "client-side" code? Thanks.