diff --git a/auxiliary/guile_containers_rbmap.c b/auxiliary/guile_containers_rbmap.c
index 0e696fc..33550ba 100644
--- a/auxiliary/guile_containers_rbmap.c
+++ b/auxiliary/guile_containers_rbmap.c
@@ -32,7 +32,7 @@ INITIALIZED_CONSTANT (_FF_ATTRIBUTE_PURE static, SCM, _rbmapi_to_pointer,

 //-------------------------------------------------------------------------
 //
-// Maps with intmax_t as keys.
+// Maps with size_t as keys.

 typedef struct rbmapi_node_s rbmapi_node_t;

@@ -76,7 +76,7 @@ scm_to_c_rbmapi (SCM map)
 }

 // Create instances of inline functions.
-VISIBLE intmax_t scm_rbmapi_iter_key (scm_t_rbmapi_iter iter);
+VISIBLE size_t scm_rbmapi_iter_key (scm_t_rbmapi_iter iter);
 VISIBLE SCM scm_rbmapi_iter_value (scm_t_rbmapi_iter iter);
 VISIBLE void scm_rbmapi_iter_set_value (scm_t_rbmapi_iter iter, SCM value);

@@ -101,7 +101,7 @@ scm_c_rbmapi_nsearch (SCM map, SCM key)
     iter = (scm_t_rbmapi_iter) rbmapi_first (tree);
   else
     {
-      const intmax_t i_key = scm_to_intmax (key);
+      const size_t i_key = scm_to_size_t (key);
       rbmapi_node_t key_node = {.key = i_key };
       iter = (scm_t_rbmapi_iter) rbmapi_nsearch (tree, &key_node);
     }
@@ -117,7 +117,7 @@ scm_c_rbmapi_psearch (SCM map, SCM key)
     iter = (scm_t_rbmapi_iter) rbmapi_last (tree);
   else
     {
-      const intmax_t i_key = scm_to_intmax (key);
+      const size_t i_key = scm_to_size_t (key);
       rbmapi_node_t key_node = {.key = i_key };
       iter = (scm_t_rbmapi_iter) rbmapi_psearch (tree, &key_node);
     }
@@ -147,7 +147,7 @@ scm_make_rbmapi (void)
 }

 static inline void
-internal_rbmapi_insert (rbmapi_t *tree, intmax_t i_key, SCM value)
+internal_rbmapi_insert (rbmapi_t *tree, size_t i_key, SCM value)
 {
   rbmapi_node_t *node = scm_gc_malloc (sizeof (rbmapi_node_t), "rbmapi_node_t");
   node->key = i_key;
@@ -159,7 +159,7 @@ VISIBLE SCM
 scm_rbmapi_set_x (SCM map, SCM key, SCM value)
 {
   rbmapi_t *tree = scm_to_c_rbmapi (map);
-  const intmax_t i_key = scm_to_intmax (key);
+  const size_t i_key = scm_to_size_t (key);
   rbmapi_node_t key_node = {.key = i_key };

   rbmapi_node_t *node = rbmapi_search (tree, &key_node);
@@ -175,7 +175,7 @@ VISIBLE SCM
 scm_rbmapi_delete_x (SCM map, SCM key)
 {
   rbmapi_t *tree = scm_to_c_rbmapi (map);
-  const intmax_t i_key = scm_to_intmax (key);
+  const size_t i_key = scm_to_size_t (key);
   rbmapi_node_t key_node = {.key = i_key };

   rbmapi_node_t *node = rbmapi_search (tree, &key_node);
@@ -192,7 +192,7 @@ scm_rbmapi_ref (SCM map, SCM key, SCM default_value)
   SCM dflt = SCM_UNBNDP (default_value) ? SCM_BOOL_F : default_value;

   rbmapi_t *tree = scm_to_c_rbmapi (map);
-  const intmax_t i_key = scm_to_intmax (key);
+  const size_t i_key = scm_to_size_t (key);
   rbmapi_node_t key_node = {.key = i_key };

   const rbmapi_node_t *node = rbmapi_search (tree, &key_node);
@@ -206,7 +206,7 @@ scm_rbmapi_fold_left (SCM proc, SCM init, SCM map, SCM start_key)
   for (scm_t_rbmapi_iter p = scm_c_rbmapi_nsearch (map, start_key); p != NULL;
        p = scm_c_rbmapi_next (map, p))
     init =
-      scm_call_3 (proc, init, scm_from_intmax (scm_rbmapi_iter_key (p)),
+      scm_call_3 (proc, init, scm_from_size_t (scm_rbmapi_iter_key (p)),
                   scm_rbmapi_iter_value (p));
   return init;
 }
@@ -217,7 +217,7 @@ scm_rbmapi_fold_right (SCM proc, SCM init, SCM map, SCM start_key)
   for (scm_t_rbmapi_iter p = scm_c_rbmapi_psearch (map, start_key); p != NULL;
        p = scm_c_rbmapi_prev (map, p))
     init =
-      scm_call_3 (proc, scm_from_intmax (scm_rbmapi_iter_key (p)),
+      scm_call_3 (proc, scm_from_size_t (scm_rbmapi_iter_key (p)),
                   scm_rbmapi_iter_value (p), init);
   return init;
 }
@@ -242,7 +242,7 @@ scm_rbmapi_to_alist (SCM map)
   // Go backwards, so consing preserves the order.
   for (scm_t_rbmapi_iter p = scm_c_rbmapi_last (map); p != NULL;
        p = scm_c_rbmapi_prev (map, p))
-    alist = scm_acons (scm_from_intmax (scm_rbmapi_iter_key (p)),
+    alist = scm_acons (scm_from_size_t (scm_rbmapi_iter_key (p)),
                        scm_rbmapi_iter_value (p), alist);

   return alist;
@@ -257,7 +257,7 @@ scm_rbmapi_map_to_list (SCM proc, SCM map)
   for (scm_t_rbmapi_iter p = scm_c_rbmapi_last (map); p != NULL;
        p = scm_c_rbmapi_prev (map, p))
     {
-      SCM element = scm_call_2 (proc, scm_from_intmax (scm_rbmapi_iter_key (p)),
+      SCM element = scm_call_2 (proc, scm_from_size_t (scm_rbmapi_iter_key (p)),
                                 scm_rbmapi_iter_value (p));
       lst = scm_cons (element, lst);
     }
@@ -270,7 +270,7 @@ scm_rbmapi_for_each (SCM proc, SCM map)
 {
   for (scm_t_rbmapi_iter p = scm_c_rbmapi_last (map); p != NULL;
        p = scm_c_rbmapi_prev (map, p))
-    scm_call_2 (proc, scm_from_intmax (scm_rbmapi_iter_key (p)),
+    scm_call_2 (proc, scm_from_size_t (scm_rbmapi_iter_key (p)),
                 scm_rbmapi_iter_value (p));
   return SCM_UNSPECIFIED;
 }
@@ -282,7 +282,7 @@ scm_rbmapi_count (SCM pred, SCM map)
   for (scm_t_rbmapi_iter p = scm_c_rbmapi_first (map); p != NULL;
        p = scm_c_rbmapi_next (map, p))
     if (scm_is_true
-        (scm_call_2 (pred, scm_from_intmax (scm_rbmapi_iter_key (p)),
+        (scm_call_2 (pred, scm_from_size_t (scm_rbmapi_iter_key (p)),
                      scm_rbmapi_iter_value (p))))
       count = scm_oneplus (count);
   return count;
diff --git a/inc/sortsmill/guile/containers/rbmap.h b/inc/sortsmill/guile/containers/rbmap.h
index 0c7ccd8..f479cec 100644
--- a/inc/sortsmill/guile/containers/rbmap.h
+++ b/inc/sortsmill/guile/containers/rbmap.h
@@ -29,7 +29,7 @@ extern "C"
 }
 #endif

-#define SCM_RBMAPI_NODE_HEAD intmax_t key; SCM value
+#define SCM_RBMAPI_NODE_HEAD size_t key; SCM value

 typedef struct
 {
@@ -38,7 +38,7 @@ typedef struct

 typedef scm_t_rbmapi_node_data *scm_t_rbmapi_iter;

-inline intmax_t scm_rbmapi_iter_key (scm_t_rbmapi_iter iter);
+inline size_t scm_rbmapi_iter_key (scm_t_rbmapi_iter iter);
 inline SCM scm_rbmapi_iter_value (scm_t_rbmapi_iter iter);
 inline void scm_rbmapi_iter_set_value (scm_t_rbmapi_iter iter, SCM value);

@@ -62,7 +62,7 @@ SCM scm_rbmapi_for_each (SCM proc, SCM map);
 SCM scm_rbmapi_count (SCM pred, SCM map);
 SCM scm_rbmapi_size (SCM map);

-inline intmax_t
+inline size_t
 scm_rbmapi_iter_key (scm_t_rbmapi_iter iter)
 {
   return iter->key;