diff --git a/lib/std/c.zig b/lib/std/c.zig
index 9543edd50e..17c826cd2f 100644
--- a/lib/std/c.zig
+++ b/lib/std/c.zig
@@ -259,7 +259,7 @@ pub usingnamespace switch (builtin.os.tag) {
 };
 
 pub extern "c" fn realloc(?*c_void, usize) ?*c_void;
-pub extern "c" fn free(*c_void) void;
+pub extern "c" fn free(?*c_void) void;
 pub extern "c" fn posix_memalign(memptr: **c_void, alignment: usize, size: usize) c_int;
 
 pub extern "c" fn futimes(fd: fd_t, times: *[2]timeval) c_int;