diff --git a/X10D/src/Drawing/Polygon.cs b/X10D/src/Drawing/Polygon.cs
index 87c29bf..f6564ad 100644
--- a/X10D/src/Drawing/Polygon.cs
+++ b/X10D/src/Drawing/Polygon.cs
@@ -1,4 +1,5 @@
-using System.Drawing;
+using System.Diagnostics.CodeAnalysis;
+using System.Drawing;
 
 namespace X10D.Drawing;
 
@@ -128,7 +129,7 @@ public class Polygon : IEquatable<Polygon>
     ///     <see langword="true" /> if <paramref name="left" /> and <paramref name="right" /> are considered not equal; otherwise,
     ///     <see langword="false" />.
     /// </returns>
-    public static bool operator !=(Polygon left, Polygon right)
+    public static bool operator !=(Polygon? left, Polygon? right)
     {
         return !(left == right);
     }
@@ -138,6 +139,7 @@ public class Polygon : IEquatable<Polygon>
     /// </summary>
     /// <param name="polygon">The polygon to convert.</param>
     /// <returns>The converted polygon.</returns>
+    [return: NotNullIfNotNull("polygon")]
     public static explicit operator Polygon?(PolygonF? polygon)
     {
         return polygon is null ? null : FromPolygonF(polygon);
diff --git a/X10D/src/Drawing/PolygonF.cs b/X10D/src/Drawing/PolygonF.cs
index 13d7813..560f984 100644
--- a/X10D/src/Drawing/PolygonF.cs
+++ b/X10D/src/Drawing/PolygonF.cs
@@ -1,4 +1,5 @@
-using System.Drawing;
+using System.Diagnostics.CodeAnalysis;
+using System.Drawing;
 using System.Numerics;
 using X10D.Numerics;
 
@@ -169,6 +170,7 @@ public class PolygonF
     /// </summary>
     /// <param name="polygon">The polygon to convert.</param>
     /// <returns>The converted polygon.</returns>
+    [return: NotNullIfNotNull("polygon")]
     public static implicit operator PolygonF?(Polygon? polygon)
     {
         return polygon is null ? null : FromPolygon(polygon);
diff --git a/X10D/src/Drawing/Polyhedron.cs b/X10D/src/Drawing/Polyhedron.cs
index 79a27cc..15538fb 100644
--- a/X10D/src/Drawing/Polyhedron.cs
+++ b/X10D/src/Drawing/Polyhedron.cs
@@ -1,4 +1,5 @@
-using System.Drawing;
+using System.Diagnostics.CodeAnalysis;
+using System.Drawing;
 using System.Numerics;
 
 namespace X10D.Drawing;
@@ -140,6 +141,7 @@ public class Polyhedron : IEquatable<Polyhedron>
     /// <returns>
     ///     The converted polyhedron, or <see langword="null" /> if <paramref name="polygon" /> is <see langword="null" />.
     /// </returns>
+    [return: NotNullIfNotNull("polygon")]
     public static implicit operator Polyhedron?(Polygon? polygon)
     {
         return polygon is null ? null : FromPolygon(polygon);
@@ -152,6 +154,7 @@ public class Polyhedron : IEquatable<Polyhedron>
     /// <returns>
     ///     The converted polyhedron, or <see langword="null" /> if <paramref name="polygon" /> is <see langword="null" />.
     /// </returns>
+    [return: NotNullIfNotNull("polygon")]
     public static implicit operator Polyhedron?(PolygonF? polygon)
     {
         return polygon is null ? null : FromPolygonF(polygon);