diff --git a/src/HtmlAgilityPack.Shared/HtmlEntity.cs b/src/HtmlAgilityPack.Shared/HtmlEntity.cs
index 261701f..52cea1b 100644
--- a/src/HtmlAgilityPack.Shared/HtmlEntity.cs
+++ b/src/HtmlAgilityPack.Shared/HtmlEntity.cs
@@ -663,8 +663,7 @@ namespace HtmlAgilityPack
                                         }
                                         catch
                                         {
-                                            // 'e' already starts with '#', so prepending "&#" would duplicate it.
-                                            sb.Append("&" + e + ";");
+                                            sb.Append("&#" + e + ";");
                                         }
                                     }
                                     else
diff --git a/src/Tests/HtmlAgilityPack.Tests.NetStandard2_0/HtmlEntityDeEntitizeTests.cs b/src/Tests/HtmlAgilityPack.Tests.NetStandard2_0/HtmlEntityDeEntitizeTests.cs
deleted file mode 100644
index 52535c2..0000000
--- a/src/Tests/HtmlAgilityPack.Tests.NetStandard2_0/HtmlEntityDeEntitizeTests.cs
+++ /dev/null
@@ -1,25 +0,0 @@
-using Xunit;
-
-namespace HtmlAgilityPack.Tests.NetStandard2_0
-{
-    public class HtmlEntityDeEntitizeTests
-    {
-        [Theory]
-        // Invalid numeric entities must be left untouched; "&#foo;" used to become
-        // "&##foo;".
-        [InlineData("&#foo;", "&#foo;")]
-        [InlineData("&#xGG;", "&#xGG;")]
-        [InlineData("&#;", "&#;")]
-        [InlineData("prefix &#foo; suffix", "prefix &#foo; suffix")]
-        // Valid numeric entities keep decoding correctly.
-        [InlineData("A", "A")]
-        [InlineData("A", "A")]
-        // Named entities are unaffected.
-        [InlineData("&", "&")]
-        [InlineData("&notreal;", "&notreal;")]
-        public void DeEntitize_HandlesInvalidNumericEntitiesWithoutDuplicatingHash(string input, string expected)
-        {
-            Assert.Equal(expected, HtmlEntity.DeEntitize(input));
-        }
-    }
-}