diff --git a/doc/external-api/README.md b/doc/external-api/README.md
index 167c02199..d3581d207 100644
--- a/doc/external-api/README.md
+++ b/doc/external-api/README.md
@@ -46,9 +46,9 @@ Nix expression `builtins.nixVersion`.
 // NOTE: This example lacks all error handling. Production code must check for
 // errors, as some return values will be undefined.
 
-void my_get_string_cb(const char * start, unsigned int n, char ** user_data)
+void my_get_string_cb(const char * start, unsigned int n, void * user_data)
 {
-    *user_data = strdup(start);
+    *((char **) user_data) = strdup(start);
 }
 
 int main()
@@ -63,7 +63,7 @@ int main()
     nix_value_force(NULL, state, value);
 
     char * version;
-    nix_get_string(NULL, value, my_get_string_cb, version);
+    nix_get_string(NULL, value, my_get_string_cb, &version);
     printf("Nix version: %s\n", version);
 
     free(version);