diff --git a/include/mruby.h b/include/mruby.h
index bca53003f..7dc1e697d 100644
--- a/include/mruby.h
+++ b/include/mruby.h
@@ -1338,7 +1338,7 @@ MRB_API mrb_value mrb_obj_clone(mrb_state *mrb, mrb_value self);
 #define TOLOWER(c) (ISUPPER(c) ? ((c) | 0x20) : (c))
 #endif
 
-MRB_API mrb_value mrb_exc_new(mrb_state *mrb, struct RClass *c, const char *ptr, size_t len);
+MRB_API mrb_value mrb_exc_new(mrb_state *mrb, struct RClass *c, const char *ptr, mrb_int len);
 MRB_API mrb_noreturn void mrb_exc_raise(mrb_state *mrb, mrb_value exc);
 
 MRB_API mrb_noreturn void mrb_raise(mrb_state *mrb, struct RClass *c, const char *msg);
diff --git a/src/error.c b/src/error.c
index faea9616d..2b23222c9 100644
--- a/src/error.c
+++ b/src/error.c
@@ -59,7 +59,7 @@ mrb_exc_new_str(mrb_state *mrb, struct RClass* c, mrb_value str)
 }
 
 MRB_API mrb_value
-mrb_exc_new(mrb_state *mrb, struct RClass *c, const char *ptr, size_t len)
+mrb_exc_new(mrb_state *mrb, struct RClass *c, const char *ptr, mrb_int len)
 {
   return mrb_exc_new_str(mrb, c, mrb_str_new(mrb, ptr, len));
 }