diff --git a/src/gasman.c b/src/gasman.c
index 4b31503bf3..36d5fc2ae4 100644
--- a/src/gasman.c
+++ b/src/gasman.c
@@ -540,7 +540,7 @@ static void CANARY_ALLOW_ACCESS_BAG(Bag bag)
     Int    bagLength = SIZE_BAG(bag);
     VALGRIND_MAKE_MEM_DEFINED(ptr, bagLength);
 
-    BagHeader * header = BAG_HEADER(bag);
+    const BagHeader * header = CONST_BAG_HEADER(bag);
     VALGRIND_MAKE_MEM_DEFINED(
         header, sizeof(*header) - sizeof(header->memory_canary_padding));
 }
@@ -554,7 +554,7 @@ static void CANARY_FORBID_ACCESS_BAG(Bag bag)
     Int    bagLength = SIZE_BAG(bag);
     VALGRIND_MAKE_MEM_NOACCESS(ptr, bagLength);
 
-    BagHeader * header = BAG_HEADER(bag);
+    const BagHeader * header = CONST_BAG_HEADER(bag);
     VALGRIND_MAKE_MEM_NOACCESS(
         header, sizeof(*header) - sizeof(header->memory_canary_padding));
 }
diff --git a/src/gasman.h b/src/gasman.h
index d5d0052fa0..ee1ab05236 100644
--- a/src/gasman.h
+++ b/src/gasman.h
@@ -108,13 +108,20 @@ enum {
 /****************************************************************************
 **
 *F  BAG_HEADER(<bag>) . . . . . . . . . . . . . . . . . . . . header of a bag
+*F  CONST_BAG_HEADER(<bag>) . . . . . . . . . . . . read-only header of a bag
 **
 **  'BAG_HEADER' returns the header of the bag with the identifier <bag>.
 */
 EXPORT_INLINE BagHeader * BAG_HEADER(Bag bag)
 {
     GAP_ASSERT(bag);
-    return (((BagHeader *)*bag) - 1);
+    return ((*(BagHeader **)bag) - 1);
+}
+
+EXPORT_INLINE const BagHeader * CONST_BAG_HEADER(Bag bag)
+{
+    GAP_ASSERT(bag);
+    return ((*(const BagHeader **)bag) - 1);
 }
 
 
@@ -136,7 +143,7 @@ EXPORT_INLINE BagHeader * BAG_HEADER(Bag bag)
 */
 EXPORT_INLINE UInt TNUM_BAG(Bag bag)
 {
-    return BAG_HEADER(bag)->type;
+    return CONST_BAG_HEADER(bag)->type;
 }
 
 
@@ -168,7 +175,7 @@ EXPORT_INLINE UInt TNUM_BAG(Bag bag)
 */
 EXPORT_INLINE uint8_t TEST_BAG_FLAG(Bag bag, uint8_t flag)
 {
-    return BAG_HEADER(bag)->flags & flag;
+    return CONST_BAG_HEADER(bag)->flags & flag;
 }
 
 EXPORT_INLINE void SET_BAG_FLAG(Bag bag, uint8_t flag)
@@ -181,6 +188,7 @@ EXPORT_INLINE void CLEAR_BAG_FLAG(Bag bag, uint8_t flag)
     BAG_HEADER(bag)->flags &= ~flag;
 }
 
+
 /****************************************************************************
 **
 *F  IS_BAG_REF(<bag>) . . . . . . verify that <bag> is a valid bag identifier
@@ -209,8 +217,9 @@ EXPORT_INLINE BOOL IS_BAG_REF(Obj bag)
 **  the size of a bag when it allocates it with 'NewBag' and may later change
 **  it with 'ResizeBag' (see "NewBag" and "ResizeBag").
 */
-EXPORT_INLINE UInt SIZE_BAG(Bag bag) {
-    return BAG_HEADER(bag)->size;
+EXPORT_INLINE UInt SIZE_BAG(Bag bag)
+{
+    return CONST_BAG_HEADER(bag)->size;
 }
 
 
@@ -223,7 +232,8 @@ EXPORT_INLINE UInt SIZE_BAG(Bag bag) {
 **  atomic operations that require a memory barrier in between dereferencing
 **  the bag pointer and accessing the contents of the bag.
 */
-EXPORT_INLINE UInt SIZE_BAG_CONTENTS(const void *ptr) {
+EXPORT_INLINE UInt SIZE_BAG_CONTENTS(const void *ptr)
+{
     return ((const BagHeader *)ptr)[-1].size;
 }
 
@@ -244,7 +254,9 @@ EXPORT_INLINE UInt SIZE_BAG_CONTENTS(const void *ptr) {
 
 /****************************************************************************
 **
-*F  PTR_BAG(<bag>)  . . . . . . . . . . . . . . . . . . . .  pointer to a bag
+*F  PTR_BAG(<bag>) . . . . . . . . . . . . . . . . . . . . . pointer to a bag
+*F  CONST_PTR_BAG(<bag>) . . . . . . . . . . . . . read-only pointer to a bag
+*F  SET_PTR_BAG(<bag>) . . . . . . . . . . . . . . . set the pointer to a bag
 **
 **  'PTR_BAG' returns the address of the data area of the bag with identifier
 **  <bag>.  Using  this pointer the application  can then  read data from the
diff --git a/src/weakptr.c b/src/weakptr.c
index f86f851f88..92e5832758 100644
--- a/src/weakptr.c
+++ b/src/weakptr.c
@@ -152,12 +152,12 @@ static inline void SET_ELM_WPOBJ(Obj list, UInt pos, Obj val)
     }
     if (!IS_BAG_REF(ptr[pos])) {
         ptr[pos] = (Bag)jl_gc_new_weakref((jl_value_t *)val);
-        jl_gc_wb_back(BAG_HEADER(list));
+        jl_gc_wb_back(CONST_BAG_HEADER(list));
     }
     else {
         jl_weakref_t * wref = (jl_weakref_t *)(ptr[pos]);
         wref->value = (jl_value_t *)val;
-        jl_gc_wb(wref, BAG_HEADER(val));
+        jl_gc_wb(wref, CONST_BAG_HEADER(val));
     }
 #endif
 }