diff --git a/src/string.cc b/src/string.cc
index 3a6c9640..4e20a2c1 100644
--- a/src/string.cc
+++ b/src/string.cc
@@ -113,7 +113,7 @@ String to_string(int val)
 String to_string(size_t val)
 {
     char buf[16];
-    sprintf(buf, "%lu", (unsigned long)val);
+    sprintf(buf, "%zu", val);
     return buf;
 }