diff --git a/Octokit.Tests/Helpers/StringExtensionsTests.cs b/Octokit.Tests/Helpers/StringExtensionsTests.cs
index f342199ef9..e4687019db 100644
--- a/Octokit.Tests/Helpers/StringExtensionsTests.cs
+++ b/Octokit.Tests/Helpers/StringExtensionsTests.cs
@@ -89,5 +89,30 @@ public void EscapeDoubleQuotesEscapesAllDoubleQuotes()
                 Assert.Equal("\\\"test milestone\\\"", input.EscapeDoubleQuotes());
             }
         }
+
+        public class EncodeSharpMethod
+        {
+            [Fact]
+            public void EncodeSharpReturnsNullForNullInput()
+            {
+                Assert.Equal(string.Empty, (null as string).EncodeSharp());
+            }
+
+            [Fact]
+            public void EncodeSharpReturnsInputWithoutDoubleQuotes()
+            {
+                string input = "some test input without double quotes in it";
+
+                Assert.Equal(input, input.EncodeSharp());
+            }
+
+            [Fact]
+            public void EncodeAllSharp()
+            {
+                string input = "#some test input with # in it#";
+
+                Assert.Equal("%23some test input with %23 in it%23", input.EncodeSharp());
+            }
+        }
     }
 }
diff --git a/Octokit/Helpers/StringExtensions.cs b/Octokit/Helpers/StringExtensions.cs
index 42ed83361f..e3d288acaf 100644
--- a/Octokit/Helpers/StringExtensions.cs
+++ b/Octokit/Helpers/StringExtensions.cs
@@ -23,8 +23,9 @@ public static bool IsNotBlank(this string value)
         public static Uri FormatUri(this string pattern, params object[] args)
         {
             Ensure.ArgumentNotNullOrEmptyString(pattern, nameof(pattern));
+            var uriString = string.Format(CultureInfo.InvariantCulture, pattern, args).EncodeSharp();
 
-            return new Uri(string.Format(CultureInfo.InvariantCulture, pattern, args), UriKind.Relative);
+            return new Uri(uriString, UriKind.Relative);
         }
 
         public static string UriEncode(this string input)
@@ -106,6 +107,11 @@ internal static string EscapeDoubleQuotes(this string value)
             return value;
         }
 
+        internal static string EncodeSharp(this string value)
+        {
+            return !string.IsNullOrEmpty(value) ? value?.Replace("#", "%23") : string.Empty;
+        }
+
         static IEnumerable<string> SplitUpperCase(this string source)
         {
             Ensure.ArgumentNotNullOrEmptyString(source, nameof(source));