diff --git a/src/ViewModels/Preference.cs b/src/ViewModels/Preference.cs
index 0dcf20a9..3d504155 100644
--- a/src/ViewModels/Preference.cs
+++ b/src/ViewModels/Preference.cs
@@ -123,7 +123,14 @@ namespace SourceGit.ViewModels {
 
         public int ExternalMergeToolType {
             get => _externalMergeToolType;
-            set => SetProperty(ref _externalMergeToolType, value);
+            set {
+                var changed = SetProperty(ref _externalMergeToolType, value);
+                if (changed && !OperatingSystem.IsWindows() && value >= 0 && value < Models.ExternalMergeTools.Supported.Count) {
+                    var tool = Models.ExternalMergeTools.Supported[value];
+                    if (File.Exists(tool.Exec)) ExternalMergeToolPath = tool.Exec;
+                    else ExternalMergeToolPath = string.Empty;
+                }
+            }
         }
 
         public string ExternalMergeToolPath {
diff --git a/src/Views/Preference.axaml.cs b/src/Views/Preference.axaml.cs
index b68649fc..212b1f96 100644
--- a/src/Views/Preference.axaml.cs
+++ b/src/Views/Preference.axaml.cs
@@ -115,14 +115,15 @@ namespace SourceGit.Views {
 
         private async void SelectExternalMergeTool(object sender, RoutedEventArgs e) {
             var type = ViewModels.Preference.Instance.ExternalMergeToolType;
-            if (type == 0 || type >= Models.ExternalMergeTools.Supported.Count) {
-                ViewModels.Preference.Instance.ExternalMergeToolType = 1;
-                type = 1;
+            if (type < 0 || type >= Models.ExternalMergeTools.Supported.Count) {
+                ViewModels.Preference.Instance.ExternalMergeToolType = 0;
+                type = 0;
             }
 
             var tool = Models.ExternalMergeTools.Supported[type];
+            var pattern = Path.GetFileName(tool.Exec);
             var options = new FilePickerOpenOptions() {
-                FileTypeFilter = [new FilePickerFileType(tool.Name) { Patterns = [tool.Exec] }],
+                FileTypeFilter = [new FilePickerFileType(tool.Name) { Patterns = [pattern] }],
                 AllowMultiple = false,
             };