diff --git a/vm/data_heap_checker.cpp b/vm/data_heap_checker.cpp
index 64e40b7487..d9cbc917d8 100644
--- a/vm/data_heap_checker.cpp
+++ b/vm/data_heap_checker.cpp
@@ -32,7 +32,7 @@ struct slot_checker {
   slot_checker(factor_vm* parent, object* obj, generation gen)
       : parent(parent), obj(obj), gen(gen) {}
 
-  void check_write_barrier(cell* slot_ptr, generation target, char mask) {
+  void check_write_barrier(cell* slot_ptr, generation target, cell mask) {
     cell object_card_pointer = parent->cards_offset + ((cell)obj >> card_bits);
     cell slot_card_pointer =
         parent->cards_offset + ((cell)slot_ptr >> card_bits);
diff --git a/vm/vm.hpp b/vm/vm.hpp
index 0bd2429fd9..0fe5d9350b 100644
--- a/vm/vm.hpp
+++ b/vm/vm.hpp
@@ -335,8 +335,8 @@ struct factor_vm {
   /* the write barrier must be called any time we are potentially storing a
      pointer from an older generation to a younger one */
   inline void write_barrier(cell* slot_ptr) {
-    *(char*)(cards_offset + ((cell)slot_ptr >> card_bits)) = card_mark_mask;
-    *(char*)(decks_offset + ((cell)slot_ptr >> deck_bits)) = card_mark_mask;
+    *(unsigned char*)(cards_offset + ((cell)slot_ptr >> card_bits)) = card_mark_mask;
+    *(unsigned char*)(decks_offset + ((cell)slot_ptr >> deck_bits)) = card_mark_mask;
   }
 
   inline void write_barrier(object* obj, cell size) {