diff --git a/mrbgems/mruby-fiber/src/fiber.c b/mrbgems/mruby-fiber/src/fiber.c
index c3a47d9a0..4fcb5cb56 100644
--- a/mrbgems/mruby-fiber/src/fiber.c
+++ b/mrbgems/mruby-fiber/src/fiber.c
@@ -10,7 +10,7 @@
 
 #define fiber_ptr(o) ((struct RFiber*)mrb_ptr(o))
 
-#define FIBER_STACK_INIT_SIZE 16
+#define FIBER_STACK_INIT_SIZE 64
 #define FIBER_CI_INIT_SIZE 4
 /* copied from vm.c */
 #define CINFO_RESUMED 3
diff --git a/mrbgems/mruby-task/include/task.h b/mrbgems/mruby-task/include/task.h
index 78db4ac9d..28e866165 100644
--- a/mrbgems/mruby-task/include/task.h
+++ b/mrbgems/mruby-task/include/task.h
@@ -87,7 +87,7 @@ typedef struct mrb_task {
 #define MRB_TIMESLICE_TICK_COUNT 3  /* Number of ticks per timeslice */
 #endif
 
-#define TASK_STACK_INIT_SIZE 16   /* Initial task stack size */
+#define TASK_STACK_INIT_SIZE 64   /* Initial task stack size */
 #define TASK_CI_INIT_SIZE 4       /* Initial task callinfo size */
 
 /*