diff --git a/ext/mri/bcrypt_pbkdf_ext.c b/ext/mri/bcrypt_pbkdf_ext.c
index df94a37..b2fd084 100644
--- a/ext/mri/bcrypt_pbkdf_ext.c
+++ b/ext/mri/bcrypt_pbkdf_ext.c
@@ -8,6 +8,7 @@ static VALUE cBCryptPbkdfEngine;
 */
 static VALUE bc_crypt_pbkdf(VALUE self, VALUE pass, VALUE salt, VALUE keylen, VALUE rounds) {
   size_t okeylen = NUM2ULONG(keylen);
+  unsigned int rounds_value = NUM2UINT(rounds);
   if (okeylen == 0 || okeylen > 1024)
     return Qnil;
   u_int8_t* okey = xmalloc(okeylen);
@@ -17,7 +18,7 @@ static VALUE bc_crypt_pbkdf(VALUE self, VALUE pass, VALUE salt, VALUE keylen, VA
     StringValuePtr(pass), RSTRING_LEN(pass),
     (const u_int8_t*)StringValuePtr(salt), RSTRING_LEN(salt),
     okey, okeylen,
-    NUM2ULONG(rounds));
+    rounds_value);
   if (ret < 0) {
     xfree(okey);
     return Qnil;
diff --git a/test/bcrypt_pnkdf/engine_test.rb b/test/bcrypt_pnkdf/engine_test.rb
index 98d990c..f4b98db 100644
--- a/test/bcrypt_pnkdf/engine_test.rb
+++ b/test/bcrypt_pnkdf/engine_test.rb
@@ -73,6 +73,15 @@ def test_invalid_pbkdf_arguments_return_nil
     assert_nil BCryptPbkdf::Engine.__bc_crypt_pbkdf('pass', 'salt', 1025, 1)
   end
 
+  def test_rounds_must_fit_the_native_range
+    assert_raises(RangeError) do
+      BCryptPbkdf::Engine.__bc_crypt_pbkdf('pass', 'salt', 32, (1 << 32) + 1)
+    end
+    assert_raises(TypeError) do
+      BCryptPbkdf::Engine.__bc_crypt_pbkdf('pass', 'salt', 32, Object.new)
+    end
+  end
+
   def test_hash_argument_lengths
     sha2pass = OpenSSL::Digest.digest('SHA512', 'pass')
     sha2salt = OpenSSL::Digest.digest('SHA512', 'salt')