diff --git a/folly/ssl/OpenSSLCertUtils.cpp b/folly/ssl/OpenSSLCertUtils.cpp
index 171c86fbb07..7ee167ac06f 100644
--- a/folly/ssl/OpenSSLCertUtils.cpp
+++ b/folly/ssl/OpenSSLCertUtils.cpp
@@ -57,12 +57,16 @@ std::string getExtOid(X509_EXTENSION* extension) {
   std::string ret(buf_size, '\0');
   auto length = OBJ_obj2txt(ret.data(), ret.size(), object, 1);
 
-  if (length > buf_size) {
+  if (length < 0) {
+    return {};
+  }
+
+  if (length >= buf_size) {
     // Reserve one byte for the terminating zero
-    ret.resize(length, '\0');
+    ret.resize(static_cast<size_t>(length) + 1, '\0');
     OBJ_obj2txt(ret.data(), ret.size(), object, 1);
   }
-  ret.resize(length);
+  ret.resize(static_cast<size_t>(length));
   return ret;
 }
 
@@ -440,4 +444,4 @@ X509StoreUniquePtr OpenSSLCertUtils::readStoreFromBuffer(ByteRange certRange) {
   return store;
 }
 } // namespace ssl
-} // namespace folly
+} // namespace folly
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