diff --git a/google/cloud/functions/internal/parse_options.cc b/google/cloud/functions/internal/parse_options.cc
index ba3a2b5..54daf29 100644
--- a/google/cloud/functions/internal/parse_options.cc
+++ b/google/cloud/functions/internal/parse_options.cc
@@ -17,6 +17,7 @@
 #include <iostream>
 #include <limits>
 #include <stdexcept>
+#include <cstdint>
 
 namespace google::cloud::functions_internal {
 FUNCTIONS_FRAMEWORK_CPP_INLINE_NAMESPACE_BEGIN