diff --git a/src/cross_sections.cpp b/src/cross_sections.cpp
index a7bd86095b..248ae9019d 100644
--- a/src/cross_sections.cpp
+++ b/src/cross_sections.cpp
@@ -283,6 +283,10 @@ void read_ce_cross_sections_xml()
 {
   // Check if cross_sections.xml exists
   const auto& filename = settings::path_cross_sections;
+  if (dir_exists(filename)) {
+    fatal_error("OPENMC_CROSS_SECTIONS is set to a directory. "
+                "It should be set to an XML file.");
+  }
   if (!file_exists(filename)) {
     // Could not find cross_sections.xml file
     fatal_error("Cross sections XML file '" + filename + "' does not exist.");