diff --git a/jit.c b/jit.c
index 7c3646fa97f..eb42aa88d02 100644
--- a/jit.c
+++ b/jit.c
@@ -108,7 +108,7 @@ rb_iseq_bare_opcode_at_pc(const rb_iseq_t *iseq, const VALUE *pc)
     return rb_vm_insn_addr2insn((const void *)at_pc);
 }
 
-unsigned long
+rb_len_t
 rb_RSTRING_LEN(VALUE str)
 {
     return RSTRING_LEN(str);
diff --git a/yjit/src/cruby_bindings.inc.rs b/yjit/src/cruby_bindings.inc.rs
index d5d6ce369ab..be6cac37f4f 100644
--- a/yjit/src/cruby_bindings.inc.rs
+++ b/yjit/src/cruby_bindings.inc.rs
@@ -1303,7 +1303,7 @@ extern "C" {
     pub fn rb_iseq_encoded_size(iseq: *const rb_iseq_t) -> ::std::os::raw::c_uint;
     pub fn rb_iseq_pc_at_idx(iseq: *const rb_iseq_t, insn_idx: u32) -> *mut VALUE;
     pub fn rb_iseq_opcode_at_pc(iseq: *const rb_iseq_t, pc: *const VALUE) -> ::std::os::raw::c_int;
-    pub fn rb_RSTRING_LEN(str_: VALUE) -> ::std::os::raw::c_ulong;
+    pub fn rb_RSTRING_LEN(str_: VALUE) -> rb_len_t;
     pub fn rb_RSTRING_PTR(str_: VALUE) -> *mut ::std::os::raw::c_char;
     pub fn rb_insn_name(insn: VALUE) -> *const ::std::os::raw::c_char;
     pub fn rb_vm_ci_argc(ci: *const rb_callinfo) -> ::std::os::raw::c_uint;
diff --git a/zjit/src/cruby_bindings.inc.rs b/zjit/src/cruby_bindings.inc.rs
index 4e130bd9830..02745e7139d 100644
--- a/zjit/src/cruby_bindings.inc.rs
+++ b/zjit/src/cruby_bindings.inc.rs
@@ -2505,7 +2505,7 @@ unsafe extern "C" {
         iseq: *const rb_iseq_t,
         pc: *const VALUE,
     ) -> ::std::os::raw::c_int;
-    pub fn rb_RSTRING_LEN(str_: VALUE) -> ::std::os::raw::c_ulong;
+    pub fn rb_RSTRING_LEN(str_: VALUE) -> rb_len_t;
     pub fn rb_RSTRING_PTR(str_: VALUE) -> *mut ::std::os::raw::c_char;
     pub fn rb_insn_name(insn: VALUE) -> *const ::std::os::raw::c_char;
     pub fn rb_vm_ci_argc(ci: *const rb_callinfo) -> ::std::os::raw::c_uint;