diff --git a/src/Flurl/Util/CommonExtensions.cs b/src/Flurl/Util/CommonExtensions.cs
index d9041854..052c032c 100644
--- a/src/Flurl/Util/CommonExtensions.cs
+++ b/src/Flurl/Util/CommonExtensions.cs
@@ -37,6 +37,10 @@ public static IEnumerable<KeyValuePair<string, object>> ToKeyValuePairs(this obj
 		/// </summary>
 		public static string ToInvariantString(this object obj) {
 			// inspired by: http://stackoverflow.com/a/19570016/62600
+				
+			var d = obj as DateTime?;
+			if (d != null)
+				return d.Value.ToString("O", CultureInfo.InvariantCulture);
 
 #if !NETSTANDARD1_0
 			var c = obj as IConvertible;
@@ -124,4 +128,4 @@ public static void Merge<TKey, TValue>(this IDictionary<TKey, TValue> d1, IDicti
 				d1.Add(kv);
 		}
 	}
-}
\ No newline at end of file
+}