diff --git a/src/Cocona.Core/ShellCompletion/Candidate/Providers/EnumCompletionCandidatesProvider.cs b/src/Cocona.Core/ShellCompletion/Candidate/Providers/EnumCompletionCandidatesProvider.cs
index bfea6a11..c6c72085 100644
--- a/src/Cocona.Core/ShellCompletion/Candidate/Providers/EnumCompletionCandidatesProvider.cs
+++ b/src/Cocona.Core/ShellCompletion/Candidate/Providers/EnumCompletionCandidatesProvider.cs
@@ -3,11 +3,14 @@ namespace Cocona.ShellCompletion.Candidate.Providers;
 public sealed class EnumCompletionCandidatesProvider : ICoconaCompletionStaticCandidatesProvider
 {
     public CompletionCandidateResult GetCandidates(CoconaCompletionCandidatesMetadata metadata)
-        => GetCandidates(metadata.ParameterType);
+    {
+        var nullableEnum = Nullable.GetUnderlyingType(metadata.ParameterType);
+        return GetCandidates(nullableEnum ?? metadata.ParameterType);
+    }
 
     private CompletionCandidateResult GetCandidates(Type type)
     {
         if (!type.IsEnum) return CompletionCandidateResult.Default;
         return CompletionCandidateResult.Keywords(Enum.GetNames(type).Select(x => new CompletionCandidateValue(x, string.Empty)));
     }
-}
\ No newline at end of file
+}