diff --git a/src/shrpx-unittest.cc b/src/shrpx-unittest.cc
index 327875bc..ee87c78a 100644
--- a/src/shrpx-unittest.cc
+++ b/src/shrpx-unittest.cc
@@ -93,7 +93,9 @@ int main(int argc, char* argv[])
                    shrpx::test_downstream_get_norm_response_header) ||
       !CU_add_test(pSuite, "util_streq", shrpx::test_util_streq) ||
       !CU_add_test(pSuite, "util_inp_strlower",
-                   shrpx::test_util_inp_strlower)) {
+                   shrpx::test_util_inp_strlower) ||
+      !CU_add_test(pSuite, "util_to_base64",
+                   shrpx::test_util_to_base64)) {
      CU_cleanup_registry();
      return CU_get_error();
    }
diff --git a/src/util.cc b/src/util.cc
index c699a2d5..0cd97af1 100644
--- a/src/util.cc
+++ b/src/util.cc
@@ -283,7 +283,9 @@ void to_base64(std::string& token68str)
       break;
     }
   }
-  token68str += std::string(4 - token68str.size() % 4, '=');
+  if(token68str.size() & 0x3) {
+    token68str.append(4 - (token68str.size() & 0x3), '=');
+  }
   return;
 }
 
diff --git a/src/util_test.cc b/src/util_test.cc
index f3b74812..ba143bc8 100644
--- a/src/util_test.cc
+++ b/src/util_test.cc
@@ -71,4 +71,15 @@ void test_util_inp_strlower(void)
   CU_ASSERT("" == a);
 }
 
+void test_util_to_base64(void)
+{
+  std::string x = "AAA--B_";
+  util::to_base64(x);
+  CU_ASSERT("AAA++B/=" == x);
+
+  x = "AAA--B_B";
+  util::to_base64(x);
+  CU_ASSERT("AAA++B/B" == x);
+}
+
 } // namespace shrpx
diff --git a/src/util_test.h b/src/util_test.h
index 3472b629..fd4ff693 100644
--- a/src/util_test.h
+++ b/src/util_test.h
@@ -29,6 +29,7 @@ namespace shrpx {
 
 void test_util_streq(void);
 void test_util_inp_strlower(void);
+void test_util_to_base64(void);
 
 } // namespace shrpx