diff --git a/federationapi/inthttp/server.go b/federationapi/inthttp/server.go
index 09590a132..02d4bc3c6 100644
--- a/federationapi/inthttp/server.go
+++ b/federationapi/inthttp/server.go
@@ -225,9 +225,12 @@ func federationClientError(err error) *api.FederationClientError {
 	if err == nil {
 		return nil
 	}
-	if ferr, ok := err.(*api.FederationClientError); ok {
+	switch ferr := err.(type) {
+	case api.FederationClientError:
+		return &ferr
+	case *api.FederationClientError:
 		return ferr
-	} else {
+	default:
 		return &api.FederationClientError{
 			Err: err.Error(),
 		}
diff --git a/internal/httputil/http.go b/internal/httputil/http.go
index 17504d691..0788ac69d 100644
--- a/internal/httputil/http.go
+++ b/internal/httputil/http.go
@@ -30,9 +30,9 @@ 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 any, errtype error](
+func PostJSON[reqtype, restype, errtype any](
 	ctx context.Context, span opentracing.Span, httpClient *http.Client,
-	apiURL string, request *reqtype, response *restype,
+	apiURL string, request *reqtype, response *restype, reserr *errtype,
 ) error {
 	jsonBytes, err := json.Marshal(request)
 	if err != nil {
@@ -71,11 +71,10 @@ func PostJSON[reqtype, restype any, errtype error](
 		return err
 	}
 	if res.StatusCode != http.StatusOK {
-		var errorBody errtype
-		if err = json.NewDecoder(res.Body).Decode(&errorBody); err != nil {
+		if err = json.NewDecoder(res.Body).Decode(reserr); err != nil {
 			return fmt.Errorf("HTTP %d from %s", res.StatusCode, apiURL)
 		}
-		return errorBody
+		return nil
 	}
 	if err = json.NewDecoder(res.Body).Decode(response); err != nil {
 		return fmt.Errorf("json.NewDecoder.Decode: %w", err)
diff --git a/internal/httputil/internalapi.go b/internal/httputil/internalapi.go
index fea7ff188..a366631f8 100644
--- a/internal/httputil/internalapi.go
+++ b/internal/httputil/internalapi.go
@@ -67,10 +67,7 @@ func MakeInternalProxyAPI[reqtype, restype any](metricsName string, f func(conte
 		if err != nil {
 			return util.JSONResponse{
 				Code: http.StatusInternalServerError,
-				JSON: &InternalAPIError{
-					Type:    reflect.TypeOf(err).String(),
-					Message: fmt.Sprintf("%s", err),
-				},
+				JSON: err,
 			}
 		}
 		return util.JSONResponse{
@@ -80,11 +77,17 @@ func MakeInternalProxyAPI[reqtype, restype any](metricsName string, f func(conte
 	})
 }
 
-func CallInternalRPCAPI[req, res any](name, url string, client *http.Client, ctx context.Context, request *req, response *res) error {
+func CallInternalRPCAPI[reqtype, restype any](name, url string, client *http.Client, ctx context.Context, request *reqtype, response *restype) error {
 	span, ctx := opentracing.StartSpanFromContext(ctx, name)
 	defer span.Finish()
 
-	return PostJSON[req, res, InternalAPIError](ctx, span, client, url, request, response)
+	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
 }
 
 func CallInternalProxyAPI[req, res any, errtype error](name, url string, client *http.Client, ctx context.Context, request *req) (res, error) {
@@ -92,5 +95,11 @@ func CallInternalProxyAPI[req, res any, errtype error](name, url string, client
 	defer span.Finish()
 
 	var response res
-	return response, PostJSON[req, res, errtype](ctx, span, client, url, request, &response)
+	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
 }