diff --git a/hphp/runtime/ext/url/ext_url.cpp b/hphp/runtime/ext/url/ext_url.cpp
index e58b63c..355ac77 100644
--- a/hphp/runtime/ext/url/ext_url.cpp
+++ b/hphp/runtime/ext/url/ext_url.cpp
@@ -205,7 +205,7 @@ static void url_encode_array(StringBuffer &ret, const Variant& varr,
 const StaticString s_arg_separator_output("arg_separator.output");
 
 Variant HHVM_FUNCTION(http_build_query, const Variant& formdata,
-                           const Variant& numeric_prefix /* = null_string */,
+                           const String& numeric_prefix /* = null_string */,
                            const String& arg_separator /* = null_string */,
                            int enc_type /* = k_PHP_QUERY_RFC1738 */) {
   if (!formdata.isArray() && !formdata.is(KindOfObject)) {
@@ -222,14 +222,8 @@ Variant HHVM_FUNCTION(http_build_query, const Variant& formdata,
 
   StringBuffer ret(1024);
   std::set<void*> seen_arrs;
-
-  String num_prefix;
-  if (!numeric_prefix.isNull())
-  {
-    num_prefix = numeric_prefix.toCStrRef();
-  }
   url_encode_array(ret, formdata, seen_arrs,
-                   num_prefix, String(), String(), arg_sep,
+                   numeric_prefix, String(), String(), arg_sep,
                    enc_type != k_PHP_QUERY_RFC3986);
   return ret.detach();
 }
diff --git a/hphp/runtime/ext/url/ext_url.h b/hphp/runtime/ext/url/ext_url.h
index 94ebe56..9339eed 100644
--- a/hphp/runtime/ext/url/ext_url.h
+++ b/hphp/runtime/ext/url/ext_url.h
@@ -43,7 +43,7 @@ Array HHVM_FUNCTION(get_meta_tags, const String& filename,
                                    bool use_include_path /* = false */);
 
 Variant HHVM_FUNCTION(http_build_query, const Variant& formdata,
-                           const Variant& numeric_prefix /* = null_string */,
+                           const String& numeric_prefix /* = null_string */,
                            const String& arg_separator /* = null_string */,
                            int enc_type = k_PHP_QUERY_RFC1738);
 Variant HHVM_FUNCTION(parse_url, const String& url,
diff --git a/hphp/runtime/ext/url/ext_url.php b/hphp/runtime/ext/url/ext_url.php
index 4c016c4..a858c12 100644
--- a/hphp/runtime/ext/url/ext_url.php
+++ b/hphp/runtime/ext/url/ext_url.php
@@ -81,7 +81,7 @@ function get_meta_tags(string $filename,
 <<__Native, __IsFoldable>>
 function http_build_query(
   mixed $query_data,
-  ?string $numeric_prefix = "",
+  string $numeric_prefix = "",
   string $arg_separator = "",
   int $enc_type = PHP_QUERY_RFC1738): mixed;
 
diff --git a/hphp/test/slow/ext_url/http_build_query_numeric_prefix.php b/hphp/test/slow/ext_url/http_build_query_numeric_prefix.php
deleted file mode 100644
index 43c3473..0000000
--- a/hphp/test/slow/ext_url/http_build_query_numeric_prefix.php
+++ /dev/null
@@ -1,14 +0,0 @@
-<?php
-$query = array(
-    0                    => "4.6.1",
-    'php'                => "7.0.99-hhvm",
-    'locale'             => "de_DE",
-    'mysql'              => "5.5.5",
-    'local_package'      => "de_DE",
-    'blogs'              => 1,
-    'users'              => 5,
-    8                    => 0,
-    'initial_db_version' => 27916,
-);
-var_dump(http_build_query($query, null, "&"));
-var_dump(http_build_query($query, 52, "&"));
diff --git a/hphp/test/slow/ext_url/http_build_query_numeric_prefix.php.expect b/hphp/test/slow/ext_url/http_build_query_numeric_prefix.php.expect
deleted file mode 100644
index 01cad12..0000000
--- a/hphp/test/slow/ext_url/http_build_query_numeric_prefix.php.expect
+++ /dev/null
@@ -1,2 +0,0 @@
-string(113) "0=4.6.1&php=7.0.99-hhvm&locale=de_DE&mysql=5.5.5&local_package=de_DE&blogs=1&users=5&8=0&initial_db_version=27916"
-string(117) "520=4.6.1&php=7.0.99-hhvm&locale=de_DE&mysql=5.5.5&local_package=de_DE&blogs=1&users=5&528=0&initial_db_version=27916"
diff --git a/hphp/test/slow/ext_url/http_build_query_numeric_prefix.php.opts b/hphp/test/slow/ext_url/http_build_query_numeric_prefix.php.opts
deleted file mode 100644
index 7dbaa00..0000000
--- a/hphp/test/slow/ext_url/http_build_query_numeric_prefix.php.opts
+++ /dev/null
@@ -1 +0,0 @@
--dhhvm.php7.all=true