diff --git a/sys/fs/constfs/constfs.c b/sys/fs/constfs/constfs.c
index cca9a7b9d..ee05b4016 100644
--- a/sys/fs/constfs/constfs.c
+++ b/sys/fs/constfs/constfs.c
@@ -241,7 +241,7 @@ static ssize_t constfs_read(vfs_file_t *filp, void *dest, size_t nbytes)
         nbytes = fp->size - filp->pos;
     }
     memcpy(dest, fp->data + filp->pos, nbytes);
-    DEBUG("constfs_read: read %d bytes\n", nbytes);
+    DEBUG("constfs_read: read %lu bytes\n", (long unsigned)nbytes);
     filp->pos += nbytes;
     return nbytes;
 }