diff --git a/hphp/runtime/base/string-util.cpp b/hphp/runtime/base/string-util.cpp
index 56d7104c14f987..51408ef85a6453 100644
--- a/hphp/runtime/base/string-util.cpp
+++ b/hphp/runtime/base/string-util.cpp
@@ -124,8 +124,8 @@ String StringUtil::Implode(const Variant& items, const String& delim,
 
   req::vector<String> sitems;
   sitems.reserve(size);
-  int len = 0;
-  int lenDelim = delim.size();
+  size_t len = 0;
+  size_t lenDelim = delim.size();
   for (ArrayIter iter(items); iter; ++iter) {
     sitems.emplace_back(iter.second().toString());
     len += sitems.back().size() + lenDelim;
diff --git a/hphp/test/slow/string_length_overflow/implode.php b/hphp/test/slow/string_length_overflow/implode.php
new file mode 100644
index 00000000000000..3d348321ac4b10
--- /dev/null
+++ b/hphp/test/slow/string_length_overflow/implode.php
@@ -0,0 +1,5 @@
+<?php
+
+$stringLarge = str_repeat('*', 300289);
+$arrayLarge = array_fill(0, 49981, '*');
+$string_implode_2 = implode($stringLarge, $arrayLarge);
diff --git a/hphp/test/slow/string_length_overflow/implode.php.expectf b/hphp/test/slow/string_length_overflow/implode.php.expectf
new file mode 100644
index 00000000000000..484c51265ce770
--- /dev/null
+++ b/hphp/test/slow/string_length_overflow/implode.php.expectf
@@ -0,0 +1,2 @@
+
+Fatal error: String length exceeded 2^31-2: 15008494201 in %s/test/slow/string_length_overflow/implode.php on line 5