diff --git a/federationapi/inthttp/client.go b/federationapi/inthttp/client.go
index 726aaa03f..812d3c6da 100644
--- a/federationapi/inthttp/client.go
+++ b/federationapi/inthttp/client.go
@@ -158,7 +158,7 @@ type getUserDevices struct {
 func (h *httpFederationInternalAPI) GetUserDevices(
 	ctx context.Context, s gomatrixserverlib.ServerName, userID string,
 ) (gomatrixserverlib.RespUserDevices, error) {
-	return httputil.CallInternalProxyAPI[getUserDevices, gomatrixserverlib.RespUserDevices, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[getUserDevices, gomatrixserverlib.RespUserDevices, *api.FederationClientError](
 		"GetUserDevices", h.federationAPIURL+FederationAPIGetUserDevicesPath, h.httpClient,
 		ctx, &getUserDevices{
 			S:      s,
@@ -175,7 +175,7 @@ type claimKeys struct {
 func (h *httpFederationInternalAPI) ClaimKeys(
 	ctx context.Context, s gomatrixserverlib.ServerName, oneTimeKeys map[string]map[string]string,
 ) (gomatrixserverlib.RespClaimKeys, error) {
-	return httputil.CallInternalProxyAPI[claimKeys, gomatrixserverlib.RespClaimKeys, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[claimKeys, gomatrixserverlib.RespClaimKeys, *api.FederationClientError](
 		"ClaimKeys", h.federationAPIURL+FederationAPIClaimKeysPath, h.httpClient,
 		ctx, &claimKeys{
 			S:           s,
@@ -192,7 +192,7 @@ type queryKeys struct {
 func (h *httpFederationInternalAPI) QueryKeys(
 	ctx context.Context, s gomatrixserverlib.ServerName, keys map[string][]string,
 ) (gomatrixserverlib.RespQueryKeys, error) {
-	return httputil.CallInternalProxyAPI[queryKeys, gomatrixserverlib.RespQueryKeys, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[queryKeys, gomatrixserverlib.RespQueryKeys, *api.FederationClientError](
 		"QueryKeys", h.federationAPIURL+FederationAPIQueryKeysPath, h.httpClient,
 		ctx, &queryKeys{
 			S:    s,
@@ -211,7 +211,7 @@ type backfill struct {
 func (h *httpFederationInternalAPI) Backfill(
 	ctx context.Context, s gomatrixserverlib.ServerName, roomID string, limit int, eventIDs []string,
 ) (gomatrixserverlib.Transaction, error) {
-	return httputil.CallInternalProxyAPI[backfill, gomatrixserverlib.Transaction, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[backfill, gomatrixserverlib.Transaction, *api.FederationClientError](
 		"Backfill", h.federationAPIURL+FederationAPIBackfillPath, h.httpClient,
 		ctx, &backfill{
 			S:        s,
@@ -232,7 +232,7 @@ type lookupState struct {
 func (h *httpFederationInternalAPI) LookupState(
 	ctx context.Context, s gomatrixserverlib.ServerName, roomID, eventID string, roomVersion gomatrixserverlib.RoomVersion,
 ) (gomatrixserverlib.RespState, error) {
-	return httputil.CallInternalProxyAPI[lookupState, gomatrixserverlib.RespState, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[lookupState, gomatrixserverlib.RespState, *api.FederationClientError](
 		"LookupState", h.federationAPIURL+FederationAPILookupStatePath, h.httpClient,
 		ctx, &lookupState{
 			S:           s,
@@ -252,7 +252,7 @@ type lookupStateIDs struct {
 func (h *httpFederationInternalAPI) LookupStateIDs(
 	ctx context.Context, s gomatrixserverlib.ServerName, roomID, eventID string,
 ) (gomatrixserverlib.RespStateIDs, error) {
-	return httputil.CallInternalProxyAPI[lookupStateIDs, gomatrixserverlib.RespStateIDs, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[lookupStateIDs, gomatrixserverlib.RespStateIDs, *api.FederationClientError](
 		"LookupStateIDs", h.federationAPIURL+FederationAPILookupStateIDsPath, h.httpClient,
 		ctx, &lookupStateIDs{
 			S:       s,
@@ -273,7 +273,7 @@ func (h *httpFederationInternalAPI) LookupMissingEvents(
 	ctx context.Context, s gomatrixserverlib.ServerName, roomID string,
 	missing gomatrixserverlib.MissingEvents, roomVersion gomatrixserverlib.RoomVersion,
 ) (res gomatrixserverlib.RespMissingEvents, err error) {
-	return httputil.CallInternalProxyAPI[lookupMissingEvents, gomatrixserverlib.RespMissingEvents, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[lookupMissingEvents, gomatrixserverlib.RespMissingEvents, *api.FederationClientError](
 		"LookupMissingEvents", h.federationAPIURL+FederationAPILookupMissingEventsPath, h.httpClient,
 		ctx, &lookupMissingEvents{
 			S:           s,
@@ -292,7 +292,7 @@ type getEvent struct {
 func (h *httpFederationInternalAPI) GetEvent(
 	ctx context.Context, s gomatrixserverlib.ServerName, eventID string,
 ) (gomatrixserverlib.Transaction, error) {
-	return httputil.CallInternalProxyAPI[getEvent, gomatrixserverlib.Transaction, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[getEvent, gomatrixserverlib.Transaction, *api.FederationClientError](
 		"GetEvent", h.federationAPIURL+FederationAPIGetEventPath, h.httpClient,
 		ctx, &getEvent{
 			S:       s,
@@ -312,7 +312,7 @@ func (h *httpFederationInternalAPI) GetEventAuth(
 	ctx context.Context, s gomatrixserverlib.ServerName,
 	roomVersion gomatrixserverlib.RoomVersion, roomID, eventID string,
 ) (gomatrixserverlib.RespEventAuth, error) {
-	return httputil.CallInternalProxyAPI[getEventAuth, gomatrixserverlib.RespEventAuth, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[getEventAuth, gomatrixserverlib.RespEventAuth, *api.FederationClientError](
 		"GetEventAuth", h.federationAPIURL+FederationAPIGetEventAuthPath, h.httpClient,
 		ctx, &getEventAuth{
 			S:           s,
@@ -340,7 +340,7 @@ type lookupServerKeys struct {
 func (h *httpFederationInternalAPI) LookupServerKeys(
 	ctx context.Context, s gomatrixserverlib.ServerName, keyRequests map[gomatrixserverlib.PublicKeyLookupRequest]gomatrixserverlib.Timestamp,
 ) ([]gomatrixserverlib.ServerKeys, error) {
-	return httputil.CallInternalProxyAPI[lookupServerKeys, []gomatrixserverlib.ServerKeys, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[lookupServerKeys, []gomatrixserverlib.ServerKeys, *api.FederationClientError](
 		"LookupServerKeys", h.federationAPIURL+FederationAPILookupServerKeysPath, h.httpClient,
 		ctx, &lookupServerKeys{
 			S:           s,
@@ -359,7 +359,7 @@ func (h *httpFederationInternalAPI) MSC2836EventRelationships(
 	ctx context.Context, s gomatrixserverlib.ServerName, r gomatrixserverlib.MSC2836EventRelationshipsRequest,
 	roomVersion gomatrixserverlib.RoomVersion,
 ) (res gomatrixserverlib.MSC2836EventRelationshipsResponse, err error) {
-	return httputil.CallInternalProxyAPI[eventRelationships, gomatrixserverlib.MSC2836EventRelationshipsResponse, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[eventRelationships, gomatrixserverlib.MSC2836EventRelationshipsResponse, *api.FederationClientError](
 		"MSC2836EventRelationships", h.federationAPIURL+FederationAPIEventRelationshipsPath, h.httpClient,
 		ctx, &eventRelationships{
 			S:       s,
@@ -378,7 +378,7 @@ type spacesReq struct {
 func (h *httpFederationInternalAPI) MSC2946Spaces(
 	ctx context.Context, dst gomatrixserverlib.ServerName, roomID string, suggestedOnly bool,
 ) (res gomatrixserverlib.MSC2946SpacesResponse, err error) {
-	return httputil.CallInternalProxyAPI[spacesReq, gomatrixserverlib.MSC2946SpacesResponse, api.FederationClientError](
+	return httputil.CallInternalProxyAPI[spacesReq, gomatrixserverlib.MSC2946SpacesResponse, *api.FederationClientError](
 		"MSC2836EventRelationships", h.federationAPIURL+FederationAPISpacesSummaryPath, h.httpClient,
 		ctx, &spacesReq{
 			S:             dst,
diff --git a/internal/httputil/http.go b/internal/httputil/http.go
index 0788ac69d..a700c9684 100644
--- a/internal/httputil/http.go
+++ b/internal/httputil/http.go
@@ -19,6 +19,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net/http"
 	"net/url"
 	"strings"
@@ -30,9 +31,9 @@ import (
 // PostJSON performs a POST request with JSON on an internal HTTP API.
 // The error will match the errtype if returned from the remote API, or
 // will be a different type if there was a problem reaching the API.
-func PostJSON[reqtype, restype, errtype any](
+func PostJSON[reqtype, restype any, errtype error](
 	ctx context.Context, span opentracing.Span, httpClient *http.Client,
-	apiURL string, request *reqtype, response *restype, reserr *errtype,
+	apiURL string, request *reqtype, response *restype,
 ) error {
 	jsonBytes, err := json.Marshal(request)
 	if err != nil {
@@ -70,14 +71,23 @@ func PostJSON[reqtype, restype, errtype any](
 	if err != nil {
 		return err
 	}
+	var body []byte
+	body, err = io.ReadAll(res.Body)
+	if err != nil {
+		return err
+	}
 	if res.StatusCode != http.StatusOK {
-		if err = json.NewDecoder(res.Body).Decode(reserr); err != nil {
+		if len(body) == 0 {
 			return fmt.Errorf("HTTP %d from %s", res.StatusCode, apiURL)
 		}
-		return nil
+		var reserr errtype
+		if err = json.Unmarshal(body, reserr); err != nil {
+			return fmt.Errorf("HTTP %d from %s", res.StatusCode, apiURL)
+		}
+		return reserr
 	}
-	if err = json.NewDecoder(res.Body).Decode(response); err != nil {
-		return fmt.Errorf("json.NewDecoder.Decode: %w", err)
+	if err = json.Unmarshal(body, response); err != nil {
+		return fmt.Errorf("json.Unmarshal: %w", err)
 	}
 	return nil
 }
diff --git a/internal/httputil/internalapi.go b/internal/httputil/internalapi.go
index a366631f8..385092d9c 100644
--- a/internal/httputil/internalapi.go
+++ b/internal/httputil/internalapi.go
@@ -81,25 +81,13 @@ func CallInternalRPCAPI[reqtype, restype any](name, url string, client *http.Cli
 	span, ctx := opentracing.StartSpanFromContext(ctx, name)
 	defer span.Finish()
 
-	var reserr *InternalAPIError
-	if err := PostJSON(ctx, span, client, url, request, response, reserr); err != nil {
-		return err
-	} else if reserr != nil {
-		return reserr
-	}
-	return nil // must be untyped nil
+	return PostJSON[reqtype, restype, InternalAPIError](ctx, span, client, url, request, response)
 }
 
-func CallInternalProxyAPI[req, res any, errtype error](name, url string, client *http.Client, ctx context.Context, request *req) (res, error) {
+func CallInternalProxyAPI[reqtype, restype any, errtype error](name, url string, client *http.Client, ctx context.Context, request *reqtype) (restype, error) {
 	span, ctx := opentracing.StartSpanFromContext(ctx, name)
 	defer span.Finish()
 
-	var response res
-	var reserr *errtype
-	if err := PostJSON(ctx, span, client, url, request, &response, &reserr); err != nil {
-		return response, err
-	} else if reserr != nil {
-		return response, *reserr
-	}
-	return response, nil // must be untyped nil
+	var response restype
+	return response, PostJSON[reqtype, restype, errtype](ctx, span, client, url, request, &response)
 }