diff --git a/vm/capi/bignum.cpp b/vm/capi/bignum.cpp
index 13b3f02507..e93d1398ff 100644
--- a/vm/capi/bignum.cpp
+++ b/vm/capi/bignum.cpp
@@ -81,7 +81,27 @@ extern "C" {
     rb_raise(rb_eArgError, "parameter is not a Bignum");
 
     return 0;
+  }
+
+  unsigned long long rb_big2ull(VALUE obj) {
+    NativeMethodEnvironment* env = NativeMethodEnvironment::get();
+
+    Object* object = env->get_object(obj);
+
+    if(object->nil_p()) {
+      rb_raise(rb_eTypeError, "no implicit conversion from nil to unsigned long");
+    } else if(Bignum* big = try_as<Bignum>(object)) {
+      if((size_t)mp_count_bits(big->mp_val()) > sizeof(unsigned long long) * CHAR_BIT)
+        rb_raise(rb_eRangeError, "bignum too big to convert into unsigned long long");
+
+      unsigned long long val = big->to_ulong_long();
+      if(big->mp_val()->sign == MP_NEG) return -val;
+      return val;
+    }
+
+    rb_raise(rb_eArgError, "parameter is not a Bignum");
 
+    return 0;
   }
 
   double rb_big2dbl(VALUE obj) {
diff --git a/vm/capi/ruby.h b/vm/capi/ruby.h
index 814434721a..54eb6aea45 100644
--- a/vm/capi/ruby.h
+++ b/vm/capi/ruby.h
@@ -795,6 +795,8 @@ double rb_num2dbl(VALUE);
 
   long long rb_big2ll(VALUE obj);
 
+  unsigned long long rb_big2ull(VALUE);
+
   double  rb_big2dbl(VALUE obj);
 
   int     rb_big_bytes_used(VALUE obj);