diff --git a/h_malloc.c b/h_malloc.c
index 8ccb3b7..ffcf0e4 100644
--- a/h_malloc.c
+++ b/h_malloc.c
@@ -1762,7 +1762,7 @@ EXPORT size_t h_malloc_usable_size(H_MALLOC_USABLE_SIZE_CONST void *arg) {
         return 0;
     }
 
-    void *p = untag_pointer((void *) (uintptr_t) arg);
+    const void *p = untag_const_pointer(arg);
 
     if (p < get_slab_region_end() && p >= ro.slab_region_start) {
         thread_unseal_metadata();
diff --git a/memtag.h b/memtag.h
index 89bff75..0ba4cbc 100644
--- a/memtag.h
+++ b/memtag.h
@@ -19,6 +19,15 @@ static inline void *untag_pointer(void *ptr) {
 #endif
 }
 
+static inline const void *untag_const_pointer(const void *ptr) {
+#ifdef HAS_ARM_MTE
+    const uintptr_t mask = UINTPTR_MAX >> 8;
+    return (const void *) ((uintptr_t) ptr & mask);
+#else
+    return ptr;
+#endif
+}
+
 static inline void *set_pointer_tag(void *ptr, u8 tag) {
 #ifdef HAS_ARM_MTE
     return (void *) (((uintptr_t) tag << 56) | (uintptr_t) untag_pointer(ptr));