diff --git a/doc/api/child_process.md b/doc/api/child_process.md
index 8d3186cbc4090d..ee8f0b4f0bc939 100644
--- a/doc/api/child_process.md
+++ b/doc/api/child_process.md
@@ -1253,9 +1253,11 @@ does not indicate that the child process has been terminated.
 added: v0.1.90
 -->
 
-* {integer}
+* {integer|undefined}
 
-Returns the process identifier (PID) of the child process.
+Returns the process identifier (PID) of the child process. If the child process
+fails to spawn due to errors, then the value is `undefined` and `error` is
+emitted.
 
 ```js
 const { spawn } = require('child_process');