diff --git a/mrbgems/mruby-method/src/method.c b/mrbgems/mruby-method/src/method.c
index 78fc63268..b0869fb35 100644
--- a/mrbgems/mruby-method/src/method.c
+++ b/mrbgems/mruby-method/src/method.c
@@ -9,7 +9,7 @@
 
 // Defined by mruby-proc-ext on which mruby-method depends
 mrb_value mrb_proc_parameters(mrb_state *mrb, mrb_value proc);
-mrb_value mrb_proc_source_location(mrb_state *mrb, struct RProc *p);
+mrb_value mrb_proc_source_location(mrb_state *mrb, const struct RProc *p);
 
 static mrb_value
 args_shift(mrb_state *mrb)
diff --git a/mrbgems/mruby-proc-binding/src/proc-binding.c b/mrbgems/mruby-proc-binding/src/proc-binding.c
index f513dafda..fd494efcd 100644
--- a/mrbgems/mruby-proc-binding/src/proc-binding.c
+++ b/mrbgems/mruby-proc-binding/src/proc-binding.c
@@ -4,7 +4,7 @@
 #include <mruby/variable.h>
 
 /* provided by mruby-proc-ext */
-mrb_value mrb_proc_source_location(mrb_state *mrb, struct RProc *p);
+mrb_value mrb_proc_source_location(mrb_state *mrb, const struct RProc *p);
 
 /* provided by mruby-binding */
 mrb_value mrb_binding_new(mrb_state *mrb, const struct RProc *proc, mrb_value recv, struct REnv *env);
diff --git a/mrbgems/mruby-proc-ext/src/proc.c b/mrbgems/mruby-proc-ext/src/proc.c
index 8713e8bb7..e896d7079 100644
--- a/mrbgems/mruby-proc-ext/src/proc.c
+++ b/mrbgems/mruby-proc-ext/src/proc.c
@@ -14,7 +14,7 @@ proc_lambda_p(mrb_state *mrb, mrb_value self)
 }
 
 mrb_value
-mrb_proc_source_location(mrb_state *mrb, struct RProc *p)
+mrb_proc_source_location(mrb_state *mrb, const struct RProc *p)
 {
   if (MRB_PROC_CFUNC_P(p)) {
     return mrb_nil_value();