diff --git a/src/ViewModels/Preference.cs b/src/ViewModels/Preference.cs
index 3d504155..a09b6530 100644
--- a/src/ViewModels/Preference.cs
+++ b/src/ViewModels/Preference.cs
@@ -125,7 +125,7 @@ namespace SourceGit.ViewModels {
             get => _externalMergeToolType;
             set {
                 var changed = SetProperty(ref _externalMergeToolType, value);
-                if (changed && !OperatingSystem.IsWindows() && value >= 0 && value < Models.ExternalMergeTools.Supported.Count) {
+                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;