diff --git a/unix_os.c b/unix_os.c
index 766a005..aafdf5d 100644
--- a/unix_os.c
+++ b/unix_os.c
@@ -265,6 +265,7 @@ fn Resulti64 osFileClose(Resulti64 handle) {
 
 fn bool osFileWriteOpenFile(Resulti64 handle, String data) {
   assert(handle.success == true);
+  assert(data.length <= data.capacity);
   i32 wrote_this_round = 0;
   for (u32 bytes_written = 0; bytes_written < data.length; bytes_written += wrote_this_round) {
     wrote_this_round = write((size_t)handle.value, data.bytes + bytes_written, data.length - bytes_written);