diff --git a/dist/tools/riotboot_gen_hdr/genhdr.c b/dist/tools/riotboot_gen_hdr/genhdr.c
index f8559d589f..409b09b487 100644
--- a/dist/tools/riotboot_gen_hdr/genhdr.c
+++ b/dist/tools/riotboot_gen_hdr/genhdr.c
@@ -49,9 +49,9 @@ int genhdr(int argc, char *argv[])
     uint8_t *hdr_buf;
 
     /* arguments storage variables */
-    long app_ver_arg = 0;
-    long start_addr_arg = 0;
-    long hdr_len_arg = 0;
+    unsigned long app_ver_arg = 0;
+    unsigned long start_addr_arg = 0;
+    unsigned long hdr_len_arg = 0;
 
     /* header variables */
     size_t hdr_len = 0;