diff --git a/apps/httpserver_raw/fs.c b/apps/httpserver_raw/fs.c
index 402c22f..5405d08 100644
--- a/apps/httpserver_raw/fs.c
+++ b/apps/httpserver_raw/fs.c
@@ -39,15 +39,15 @@
 
 /*-----------------------------------------------------------------------------------*/
 int
-fs_open(char *name, struct fs_file *file)
+fs_open(const char *name, struct fs_file *file)
 {
-  struct fsdata_file_noconst *f;
+  const struct fsdata_file *f;
 
-  for(f = (struct fsdata_file_noconst *)FS_ROOT;
+  for(f = FS_ROOT;
       f != NULL;
-      f = (struct fsdata_file_noconst *)f->next) {
-    if (!strcmp(name, (char *)f->name)) {
-      file->data = (char *)f->data;
+      f = f->next) {
+    if (!strcmp(name, (const char*)f->name)) {
+      file->data = f->data;
       file->len = f->len;
       return 1;
     }
diff --git a/apps/httpserver_raw/fs.h b/apps/httpserver_raw/fs.h
index 6c4eb10..1f7d434 100644
--- a/apps/httpserver_raw/fs.h
+++ b/apps/httpserver_raw/fs.h
@@ -33,12 +33,12 @@
 #define __FS_H__
 
 struct fs_file {
-  char *data;
+  const unsigned char *data;
   int len;
 };
 
 /* file must be allocated by caller and will be filled in
    by the function. */
-int fs_open(char *name, struct fs_file *file);
+int fs_open(const char *name, struct fs_file *file);
 
 #endif /* __FS_H__ */
diff --git a/apps/httpserver_raw/fsdata.h b/apps/httpserver_raw/fsdata.h
index ee4947b..a6c5262 100644
--- a/apps/httpserver_raw/fsdata.h
+++ b/apps/httpserver_raw/fsdata.h
@@ -39,11 +39,4 @@ struct fsdata_file {
   const int len;
 };
 
-struct fsdata_file_noconst {
-  struct fsdata_file *next;
-  unsigned char *name;
-  unsigned char *data;
-  int len;
-};
-
 #endif /* __FSDATA_H__ */
diff --git a/apps/httpserver_raw/httpd.c b/apps/httpserver_raw/httpd.c
index eb0ef26..e13d8d0 100644
--- a/apps/httpserver_raw/httpd.c
+++ b/apps/httpserver_raw/httpd.c
@@ -41,8 +41,8 @@
 #include "fs.h"
 
 struct http_state {
-  char *file;
-  u16_t left;
+  u32_t left;
+  const unsigned char *file;
   u8_t retries;
 };
 
@@ -80,6 +80,7 @@ send_data(struct tcp_pcb *pcb, struct http_state *hs)
     len = tcp_sndbuf(pcb);
   } else {
     len = hs->left;
+    LWIP_ASSERT((len == hs->left), "hs->left did not fit into u16_t!");
   }
 
   do {
@@ -176,6 +177,7 @@ http_recv(void *arg, struct tcp_pcb *pcb, struct pbuf *p, err_t err)
         }
 
         hs->file = file.data;
+        LWIP_ASSERT((file.len >= 0), "File length must be positive!");
         hs->left = file.len;
         /* printf("data %p len %ld\n", hs->file, hs->left);*/