diff --git a/docs/conceptual/expr.substitute-method-[fsharp].md b/docs/conceptual/expr.substitute-method-[fsharp].md
index d2850108..0d118b34 100644
--- a/docs/conceptual/expr.substitute-method-[fsharp].md
+++ b/docs/conceptual/expr.substitute-method-[fsharp].md
@@ -32,7 +32,7 @@ expr.Substitute (substitution)
 
 #### Parameters
 *substitution*
-Type: [Var](https://msdn.microsoft.com/library/2b1237f9-d897-4bcf-872a-4a297db3f7b5)**->**[Expr](https://msdn.microsoft.com/library/ed6a2caf-69d4-45c2-ab97-e9b3be9bce65)
+Type: [Var](https://msdn.microsoft.com/library/2b1237f9-d897-4bcf-872a-4a297db3f7b5)**->**[Expr](https://msdn.microsoft.com/library/ed6a2caf-69d4-45c2-ab97-e9b3be9bce65) option
 
 
 The function to map variables into expressions.
@@ -53,4 +53,4 @@ Supported in: 2.0, 4.0, Portable
 ## See Also
 [Quotations.Expr Class (F#)](Quotations.Expr-Class-%5BFSharp%5D.md)
 
-[Microsoft.FSharp.Quotations Namespace (F#)](Microsoft.FSharp.Quotations-Namespace-%5BFSharp%5D.md)
\ No newline at end of file
+[Microsoft.FSharp.Quotations Namespace (F#)](Microsoft.FSharp.Quotations-Namespace-%5BFSharp%5D.md)