diff --git a/src/dsp/cpu.c b/src/dsp/cpu.c
index 2a78a18e..88d42623 100644
--- a/src/dsp/cpu.c
+++ b/src/dsp/cpu.c
@@ -9,6 +9,8 @@
 //
 // Author: Christian Duvivier (cduvivier@google.com)
 
+#include <stddef.h>  // for NULL
+
 #include "./dsp.h"
 
 #if defined(__cplusplus) || defined(c_plusplus)