diff --git a/lib/layouts.js b/lib/layouts.js
index 9a15317..aac5144 100644
--- a/lib/layouts.js
+++ b/lib/layouts.js
@@ -249,7 +249,7 @@ function patternLayout(pattern, tokens, timezoneOffset) {
   }
 
   function pid(loggingEvent) {
-    return loggingEvent && loggingEvent.pid ? loggingEvent.pid : process.pid;
+    return loggingEvent && loggingEvent.pid ? loggingEvent.pid.toString() : process.pid.toString();
   }
 
   function clusterInfo(loggingEvent, specifier) {