diff --git a/ruma-api-macros/src/api.rs b/ruma-api-macros/src/api.rs
index ae672d7e..dea6d079 100644
--- a/ruma-api-macros/src/api.rs
+++ b/ruma-api-macros/src/api.rs
@@ -120,7 +120,7 @@ pub fn expand_all(api: Api) -> syn::Result<TokenStream> {
     let mut header_kvs = api.request.append_header_kvs();
     if authentication == "AccessToken" {
         header_kvs.extend(quote! {
-            req_builder = req_builder.header(
+            req_headers.insert(
                 #http::header::AUTHORIZATION,
                 #http::header::HeaderValue::from_str(
                     &::std::format!(
@@ -332,7 +332,12 @@ pub fn expand_all(api: Api) -> syn::Result<TokenStream> {
                         },
                         #request_path_string,
                         #request_query_string,
-                    ));
+                    ))
+                    .header(#ruma_api::exports::http::header::CONTENT_TYPE, "application/json");
+
+                let mut req_headers = req_builder
+                    .headers_mut()
+                    .expect("`http::RequestBuilder` is in unusable state");
 
                 #header_kvs
 
diff --git a/ruma-api-macros/src/api/request.rs b/ruma-api-macros/src/api/request.rs
index a52d92de..82265615 100644
--- a/ruma-api-macros/src/api/request.rs
+++ b/ruma-api-macros/src/api/request.rs
@@ -64,7 +64,7 @@ impl Request {
                     {
                         quote! {
                             if let Some(header_val) = self.#field_name.as_ref() {
-                                req_builder = req_builder.header(
+                                req_headers.insert(
                                     #http::header::#header_name,
                                     #http::header::HeaderValue::from_str(header_val)?,
                                 );
@@ -72,7 +72,7 @@ impl Request {
                         }
                     }
                     _ => quote! {
-                        req_builder = req_builder.header(
+                        req_headers.insert(
                             #http::header::#header_name,
                             #http::header::HeaderValue::from_str(self.#field_name.as_ref())?,
                         );
diff --git a/ruma-api/tests/header_override.rs b/ruma-api/tests/header_override.rs
index db7c4a80..b96b8a07 100644
--- a/ruma-api/tests/header_override.rs
+++ b/ruma-api/tests/header_override.rs
@@ -1,7 +1,7 @@
 use std::convert::TryFrom;
 
 use http::header::{Entry, CONTENT_TYPE};
-use ruma_api::ruma_api;
+use ruma_api::{ruma_api, OutgoingRequest as _};
 
 ruma_api! {
     metadata: {
@@ -16,6 +16,9 @@ ruma_api! {
     request: {
         #[ruma_api(header = LOCATION)]
         pub location: Option<String>,
+
+        #[ruma_api(header = CONTENT_TYPE)]
+        pub stuff: String,
     }
 
     response: {
@@ -25,7 +28,7 @@ ruma_api! {
 }
 
 #[test]
-fn content_type_override() {
+fn response_content_type_override() {
     let res = Response { stuff: "magic".into() };
     let mut http_res = http::Response::<Vec<u8>>::try_from(res).unwrap();
 
@@ -40,3 +43,18 @@ fn content_type_override() {
     );
     assert_eq!(http_res.headers().get("content-type").unwrap(), "magic");
 }
+
+#[test]
+fn request_content_type_override() {
+    let req = Request { location: None, stuff: "magic".into() };
+    let mut http_req = req.try_into_http_request("https://homeserver.tld", None).unwrap();
+
+    assert_eq!(
+        match http_req.headers_mut().entry(CONTENT_TYPE) {
+            Entry::Occupied(occ) => occ.iter().count(),
+            _ => 0,
+        },
+        1
+    );
+    assert_eq!(http_req.headers().get("content-type").unwrap(), "magic");
+}