diff --git a/ml_io.cpp b/ml_io.cpp
index 0fc3e7f..1889b65 100644
--- a/ml_io.cpp
+++ b/ml_io.cpp
@@ -36,7 +36,7 @@ MlValue list_dir(const std::string &path) {
 
 	DIR *dirp = opendir(path.c_str());
 	if (dirp == NULL) {
-		// handle error - probably not a dir or non existing path
+		throw std::runtime_error("could not open dir (" + path + ")");
 	}
 	struct dirent *dp;
 	while ((dp = readdir(dirp)) != NULL) {