diff --git a/openrpc/types/types.go b/openrpc/types/types.go
index 83cf8caf823d42c3e8a3be2d292200bf355f5aa1..2cf52899665b14d4e527b4a26c8528b600867dfe 100644
--- a/openrpc/types/types.go
+++ b/openrpc/types/types.go
@@ -93,7 +93,7 @@ type Tag struct {
 
 type ExternalDocumentation struct {
 	Description string `json:"description,omitempty"`
-	URL         string `json:"url"`
+	URL         string `json:"url,omitempty"`
 }
 
 type ExamplePairing struct {