Fallback for enum.Format(G) when not valid (#10610)

diff --git a/src/mscorlib/src/System/Enum.cs b/src/mscorlib/src/System/Enum.cs
index 2403b6a..489f367 100644 (file)
--- a/src/mscorlib/src/System/Enum.cs
+++ b/src/mscorlib/src/System/Enum.cs
@@ -688,7 +688,7 @@ namespace System
 
             char formatCh = format[0];
             if (formatCh == 'G' || formatCh == 'g')
-                return GetEnumName(rtType, ToUInt64(value));
+                return GetEnumName(rtType, ToUInt64(value)) ?? value.ToString();
 
             if (formatCh == 'D' || formatCh == 'd')
                 return value.ToString();