diff --git a/shape.c b/shape.c
index 4cd4acd7f9..8d8314db33 100644
--- a/shape.c
+++ b/shape.c
@@ -1233,6 +1233,14 @@ Init_default_shapes(void)
     rb_shape_tree_ptr->shape_cache = (redblack_node_t *)mmap(NULL, rb_size_mul_or_raise(REDBLACK_CACHE_SIZE, sizeof(redblack_node_t), rb_eRuntimeError),
                          PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
     rb_shape_tree_ptr->cache_size = 0;
+
+    // If mmap fails, then give up on the redblack tree cache.
+    // We set the cache size such that the redblack node allocators think
+    // the cache is full.
+    if (GET_SHAPE_TREE()->shape_cache == MAP_FAILED) {
+        GET_SHAPE_TREE()->shape_cache = 0;
+        GET_SHAPE_TREE()->cache_size = REDBLACK_CACHE_SIZE;
+    }
 #endif
 
     // Shapes by size pool