diff --git a/api/api.go b/api/api.go
index 74335d4..2147651 100644
--- a/api/api.go
+++ b/api/api.go
@@ -189,7 +189,7 @@ func (api *API) registerWebAppRoutes(router *gin.Engine) {
 }
 
 func (api *API) registerKOAPIRoutes(apiGroup *gin.RouterGroup) {
-	koGroup := apiGroup.Group("/ko")
+	koGroup := apiGroup.Group("/ko", koJSONMiddleware)
 
 	// KO sync routes (webapp uses - progress & activity)
 	koGroup.GET("/documents/:document/file", api.authKOMiddleware, api.createDownloadDocumentHandler(apiErrorPage))
@@ -325,6 +325,13 @@ func (api *API) loadTemplates(
 	return nil
 }
 
+func (api *API) templateMiddleware(router *gin.Engine) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		router.HTMLRender = *api.generateTemplates()
+		c.Next()
+	}
+}
+
 func loggingMiddleware(c *gin.Context) {
 	// Start timer
 	startTime := time.Now()
@@ -361,9 +368,12 @@ func loggingMiddleware(c *gin.Context) {
 	log.WithFields(logData).Info(fmt.Sprintf("%s %s", c.Request.Method, c.Request.URL.Path))
 }
 
-func (api *API) templateMiddleware(router *gin.Engine) gin.HandlerFunc {
-	return func(c *gin.Context) {
-		router.HTMLRender = *api.generateTemplates()
-		c.Next()
+// koJSONMiddleware forces JSON Content-Type to only return `application/json`. This is addressing
+// the following issue: https://github.com/koreader/koreader/issues/13629
+func koJSONMiddleware(c *gin.Context) {
+	c.Next()
+	contentType := c.Writer.Header().Get("Content-Type")
+	if strings.HasPrefix(contentType, "application/json") {
+		c.Header("Content-Type", "application/json")
 	}
 }