diff --git a/src/UglyToad.PdfPig/PdfFonts/Cmap/CMapUtils.cs b/src/UglyToad.PdfPig/PdfFonts/Cmap/CMapUtils.cs
index d5c0fd6fc..26e1dbad5 100644
--- a/src/UglyToad.PdfPig/PdfFonts/Cmap/CMapUtils.cs
+++ b/src/UglyToad.PdfPig/PdfFonts/Cmap/CMapUtils.cs
@@ -16,7 +16,7 @@ public static int ToInt(this ReadOnlySpan<byte> data)
             return code;
         }
 
-        public static void PutAll<TKey, TValue>(this Dictionary<TKey, TValue> target, IReadOnlyDictionary<TKey, TValue> source)
+        public static void PutAll<TKey, TValue>(this Dictionary<TKey, TValue> target, IReadOnlyDictionary<TKey, TValue> source) where TKey : notnull
         {
             foreach (var pair in source)
             {
diff --git a/src/UglyToad.PdfPig/Util/StackDictionary.cs b/src/UglyToad.PdfPig/Util/StackDictionary.cs
index 4b91e06ce..4b521439b 100644
--- a/src/UglyToad.PdfPig/Util/StackDictionary.cs
+++ b/src/UglyToad.PdfPig/Util/StackDictionary.cs
@@ -5,7 +5,7 @@ namespace UglyToad.PdfPig.Util
     using System.Collections.Generic;
     using System.Diagnostics.CodeAnalysis;
 
-    internal class StackDictionary<K, V>
+    internal class StackDictionary<K, V> where K : notnull
     {
         private readonly List<Dictionary<K, V>> values = new List<Dictionary<K, V>>();