diff --git a/src/Duration.cpp b/src/Duration.cpp
index 0d7b0bbfe..31fd3b09b 100644
--- a/src/Duration.cpp
+++ b/src/Duration.cpp
@@ -292,7 +292,7 @@ std::string Duration::formatISO () const
   }
   else
   {
-    return "P0S";
+    return "PT0S";
   }
 }
 
diff --git a/src/Variant.cpp b/src/Variant.cpp
index 7f21b53b2..d271e5bcf 100644
--- a/src/Variant.cpp
+++ b/src/Variant.cpp
@@ -1979,7 +1979,7 @@ Variant::operator std::string () const
       }
       else
       {
-        return "P0S";
+        return "PT0S";
       }
     }