diff --git a/gammapkt.cc b/gammapkt.cc
index 3f3c2a012..19a619806 100644
--- a/gammapkt.cc
+++ b/gammapkt.cc
@@ -213,7 +213,10 @@ void init_photoion_data() {
   for (int Z = 0; Z < numb_xcom_elements; Z++) {
     photoion_data[Z].reserve(100);
   }
-  std::ifstream data_fs("xcom_photoion_data.txt");
+  const std::string filepath{"xcom_photoion_data.txt"};
+  assert_always(std::filesystem::exists(filepath));
+
+  std::ifstream data_fs(filepath);
   std::string line_str;
   // now read the file a second time to store the data
   while (getline(data_fs, line_str)) {