diff --git a/MaybeError/MaybeError.csproj b/MaybeError/MaybeError.csproj
index ea3c177..203d4d7 100644
--- a/MaybeError/MaybeError.csproj
+++ b/MaybeError/MaybeError.csproj
@@ -8,7 +8,7 @@
 
 	<PropertyGroup>
 		<PackageId>MaybeError</PackageId>
-		<Version>1.0.3</Version>
+		<Version>1.0.4</Version>
 		<Authors>Amatsugu</Authors>
 		<PackageDescription>Errors as values</PackageDescription>
 		<RepositoryUrl>https://github.com/Amatsugu/MaybeError</RepositoryUrl>
diff --git a/MaybeError/ValueMaybe.cs b/MaybeError/ValueMaybe.cs
index e29fce5..9c91044 100644
--- a/MaybeError/ValueMaybe.cs
+++ b/MaybeError/ValueMaybe.cs
@@ -9,6 +9,7 @@ public interface IValueMaybe<T, E> where T : struct where E : Error
 {
 	[MemberNotNullWhen(true, nameof(Error))]
 	bool HasError { get; }
+	[MemberNotNullWhen(false, nameof(Error))]
 	bool HasValue => !HasError;
 	E? Error { get; }
 	T Value { get; }
@@ -18,6 +19,7 @@ public readonly struct ValueMaybe<T, E> : IValueMaybe<T, E> where T : struct whe
 {
 	[MemberNotNullWhen(true, nameof(Error))]
 	public readonly bool HasError { get; }
+	[MemberNotNullWhen(false, nameof(Error))]
 	public readonly bool HasValue => !HasError;
 	public readonly E? Error { get; }
 	public readonly T Value => HasError ? throw Error.GetException() : _value;