diff --git a/cont.c b/cont.c
index ab3f9b5085..996328c554 100644
--- a/cont.c
+++ b/cont.c
@@ -130,7 +130,7 @@ cont_free(void *ptr)
 static size_t
 cont_memsize(const void *ptr)
 {
-    rb_context_t *cont = ptr;
+    const rb_context_t *cont = ptr;
     size_t size = 0;
     if (cont) {
 	size = sizeof(*cont);
@@ -208,7 +208,7 @@ fiber_free(void *ptr)
 static size_t
 fiber_memsize(const void *ptr)
 {
-    rb_fiber_t *fib = ptr;
+    const rb_fiber_t *fib = ptr;
     size_t size = 0;
     if (ptr) {
 	size = sizeof(*fib);