diff --git a/src/perf_counters.cc b/src/perf_counters.cc
index d466e27e..2eb97eb4 100644
--- a/src/perf_counters.cc
+++ b/src/perf_counters.cc
@@ -39,7 +39,8 @@ size_t PerfCounterValues::Read(const std::vector<int>& leaders) {
     auto read_bytes = ::read(lead, ptr, size);
     if (read_bytes >= ssize_t(sizeof(uint64_t))) {
       // Actual data bytes are all bytes minus initial padding
-      std::size_t data_bytes = read_bytes - sizeof(uint64_t);
+      std::size_t data_bytes =
+          static_cast<std::size_t>(read_bytes) - sizeof(uint64_t);
       // This should be very cheap since it's in hot cache
       std::memmove(ptr, ptr + sizeof(uint64_t), data_bytes);
       // Increment our counters