diff --git a/rtgui/options.cc b/rtgui/options.cc
index af40595aa..a6756d37d 100644
--- a/rtgui/options.cc
+++ b/rtgui/options.cc
@@ -1269,30 +1269,24 @@ void Options::readFromFile(Glib::ustring fname)
                 }
 
                 // check and add extensions that are missing from config
-                static const std::vector<std::string> extensions_known = {
-                    "3fr", "arw", "arq", "cr2",  "cr3", "crf", "crw",  "dcr", "dng",
-                    "fff", "iiq", "jpg", "jpeg", "jxl", "kdc", "mef",  "mos", "mrw",
-                    "nef", "nrw", "orf", "ori",  "pef", "png", "raf",  "raw", "rw2",
-                    "rwl", "rwz", "sr2", "srf",  "srw", "tif", "tiff", "x3f"};
-
-                std::map<std::string, int> extensions_checked;
+                std::map<std::string, int> checkedExtensions;
 
                 if (parseExtensions.size() == parseExtensionsEnabled.size()) {
                     for (auto i = 0; i < parseExtensions.size(); ++i) {
-                        extensions_checked[parseExtensions[i]] = parseExtensionsEnabled[i];
+                        checkedExtensions[parseExtensions[i]] = parseExtensionsEnabled[i];
                     }
                 }
 
                 parseExtensions.clear();
                 parseExtensionsEnabled.clear();
 
-                for (auto const &i : extensions_known) {
-                    if (extensions_checked.count(i) == 0) {
-                        extensions_checked[i] = 1;
+                for (auto const &i : knownExtensions) {
+                    if (checkedExtensions.count(i) == 0) {
+                        checkedExtensions[i] = 1;
                     }
                 }
 
-                for (auto const &x : extensions_checked) {
+                for (auto const &x : checkedExtensions) {
                     parseExtensions.emplace_back(x.first);
                     parseExtensionsEnabled.emplace_back(x.second);
                 }
diff --git a/rtgui/options.h b/rtgui/options.h
index 38f4fe896..6a2b0c35d 100644
--- a/rtgui/options.h
+++ b/rtgui/options.h
@@ -310,6 +310,13 @@ public:
     int maxThumbnailWidth;
     std::size_t maxCacheEntries;
     int thumbInterp; // 0: nearest, 1: bilinear
+
+    std::vector<std::string> knownExtensions = {
+        "3fr", "arw", "arq", "cr2",  "cr3", "crf", "crw",  "dcr", "dng",
+        "fff", "iiq", "jpg", "jpeg", "jxl", "kdc", "mef",  "mos", "mrw",
+        "nef", "nrw", "orf", "ori",  "pef", "png", "raf",  "raw", "rw2",
+        "rwl", "rwz", "sr2", "srf",  "srw", "tif", "tiff", "x3f"};
+
     std::vector<Glib::ustring> parseExtensions;   // List containing all extensions type
     std::vector<int> parseExtensionsEnabled;      // List of bool to retain extension or not
     std::vector<Glib::ustring> parsedExtensions;  // List containing all retained extensions (lowercase)
diff --git a/rtgui/preferences.cc b/rtgui/preferences.cc
index 04a9ece60..97bfb92b6 100644
--- a/rtgui/preferences.cc
+++ b/rtgui/preferences.cc
@@ -2687,8 +2687,29 @@ void Preferences::addExtPressed()
 
 void Preferences::delExtPressed()
 {
+    const Glib::RefPtr<Gtk::TreeSelection> selection = extensions->get_selection();
 
-    extensionModel->erase(extensions->get_selection()->get_selected());
+    if (!selection) {
+        return;
+    }
+
+    const Gtk::TreeModel::iterator selected = selection->get_selected();
+
+    if (!selected) {
+        return;
+    }
+
+    bool delOkay = true;
+    for (auto const &x : moptions.knownExtensions) {
+        if (x == (*selected)[extensionColumns.ext]) {
+            delOkay = false;
+            break;
+        }
+    }
+
+    if (delOkay) {
+        extensionModel->erase(selected);
+    }
 }
 
 void Preferences::moveExtUpPressed()