diff --git a/Octokit/Helpers/StringExtensions.cs b/Octokit/Helpers/StringExtensions.cs
index 30526f831f..42ed83361f 100644
--- a/Octokit/Helpers/StringExtensions.cs
+++ b/Octokit/Helpers/StringExtensions.cs
@@ -73,6 +73,14 @@ public static Uri ExpandUriTemplate(this string template, object values)
         public static string ToRubyCase(this string propertyName)
         {
             Ensure.ArgumentNotNullOrEmptyString(propertyName, nameof(propertyName));
+
+            // If the entire property is already all upper case, then do not split it across
+            // word boundaries. For example, "UBUNTU" should not be changed to "u_b_u_n_t_u".
+            if (string.Equals(propertyName, propertyName.ToUpperInvariant(), StringComparison.Ordinal))
+            {
+                return propertyName;
+            }
+
             return string.Join("_", propertyName.SplitUpperCase()).ToLowerInvariant();
         }