diff --git a/src/libexpr-c/nix_api_external.h b/src/libexpr-c/nix_api_external.h
index a52346da9..6c524b975 100644
--- a/src/libexpr-c/nix_api_external.h
+++ b/src/libexpr-c/nix_api_external.h
@@ -48,7 +48,7 @@ void nix_set_string_return(nix_string_return * str, const char * c);
  * Print to the nix_printer
  *
  * @param[out] context Optional, stores error information
- * @param printer The nix_printer to print to
+ * @param[out] printer The nix_printer to print to
  * @param[in] str The string to print
  * @returns NIX_OK if everything worked
  */