diff --git a/vm/vm.hpp b/vm/vm.hpp
index bb7908be28..fec5b38a82 100644
--- a/vm/vm.hpp
+++ b/vm/vm.hpp
@@ -544,7 +544,7 @@ struct factor_vm {
   // io
   void init_c_io();
   void io_error_if_not_EINTR();
-  FILE* safe_fopen(char* filename, char* mode);
+  FILE* safe_fopen(char* filename, const char* mode);
   int safe_fgetc(FILE* stream);
   size_t safe_fread(void* ptr, size_t size, size_t nitems, FILE* stream);
   void safe_fputc(int c, FILE* stream);