diff --git a/urandom_entropy_src.cpp b/urandom_entropy_src.cpp
index 83f3c70..c311cb5 100644
--- a/urandom_entropy_src.cpp
+++ b/urandom_entropy_src.cpp
@@ -3,6 +3,8 @@
 
 #include "urandom_entropy_src.h"
 
+#include <cstdint>
+
 namespace fortuna {
 namespace accumulator {
 
@@ -23,7 +25,7 @@ auto UrandomEntropySrc::event(accumulator::EventAdderImpl adder) -> void {
 		if (urandom) {
 			urandom.read(
 				reinterpret_cast<char*>(&UrandomEntropySrc::bytes),
-				reinterpret_cast<uint64_t>(UrandomEntropySrc::bytes.size()));
+				static_cast<int64_t>(reinterpret_cast<uint64_t>(UrandomEntropySrc::bytes.size())));
 			urandom.close();
 		}
 		else {