diff --git a/debug_counter.c b/debug_counter.c
index f17b3b4d7c..60cc2c3ac1 100644
--- a/debug_counter.c
+++ b/debug_counter.c
@@ -44,14 +44,14 @@ rb_debug_counter_show_results(const char *msg)
 }
 
 VALUE
-rb_debug_counter_show(void)
+rb_debug_counter_show(RB_UNUSED_VAR(VALUE klass))
 {
     rb_debug_counter_show_results("method call");
     return Qnil;
 }
 
 VALUE
-rb_debug_counter_reset(void)
+rb_debug_counter_reset(RB_UNUSED_VAR(VALUE klass))
 {
     for (int i = 0; i < RB_DEBUG_COUNTER_MAX; i++) {
         switch (i) {
diff --git a/debug_counter.h b/debug_counter.h
index 3fa644828d..2add2acc25 100644
--- a/debug_counter.h
+++ b/debug_counter.h
@@ -349,8 +349,8 @@ rb_debug_counter_add(enum rb_debug_counter_type type, int add, int cond)
     return cond;
 }
 
-VALUE rb_debug_counter_reset(void);
-VALUE rb_debug_counter_show(void);
+VALUE rb_debug_counter_reset(VALUE klass);
+VALUE rb_debug_counter_show(VALUE klass);
 
 #define RB_DEBUG_COUNTER_INC(type)                rb_debug_counter_add(RB_DEBUG_COUNTER_##type, 1, 1)
 #define RB_DEBUG_COUNTER_INC_UNLESS(type, cond) (!rb_debug_counter_add(RB_DEBUG_COUNTER_##type, 1, !(cond)))