diff --git a/Doc/ext/extending.tex b/Doc/ext/extending.tex
index bc2f9cb8fcb..7016f94d4ce 100644
--- a/Doc/ext/extending.tex
+++ b/Doc/ext/extending.tex
@@ -1306,7 +1306,7 @@ extern "C" {
 /* C API functions */
 #define PySpam_System_NUM 0
 #define PySpam_System_RETURN int
-#define PySpam_System_PROTO (char *command)
+#define PySpam_System_PROTO (const char *command)
 
 /* Total number of C API pointers */
 #define PySpam_API_pointers 1