diff --git a/spec/Candid.md b/spec/Candid.md
index 72f0c4d3..29859d7e 100644
--- a/spec/Candid.md
+++ b/spec/Candid.md
@@ -791,7 +791,7 @@ not (<datatype> <: opt <datatype'>)
 ---------------------------------
 opt <datatype> <: opt <datatype'>
 
-not (null <: <datatype>)
+not (null <: <datatype'>)
 not (<datatype> <: <datatype'>)
 ---------------------------------
 opt <datatype> <: opt <datatype'>
@@ -915,7 +915,7 @@ C[vec <t> <: vec <t'>]( vec { <v>;* } ) = vec { C[<t> <: <t'>](<v>);* }
 
 #### Options
 
-The null type coerces into any option type:
+The null type and the reserved type coerce into any option type:
 ```
 C[null <: opt <t>](null) = null
 ```
@@ -927,15 +927,20 @@ C[opt <t> <: opt <t'>](opt <v>) = opt C[<t> <: <t'>](v)  if <t> <: <t'>
 C[opt <t> <: opt <t'>](opt <v>) = null                   if not(<t> <: <t'>)
 ```
 
-NOTE: These rules above imply that a Candid decoder has to be able to decide the subtyping relation at runtime.
+Coercing a non-null, non-optional and non-reserved type at an option type treats it as an optional value, if it has a suitable type:
+```
+C[<t> <: opt <t'>](<v>) = opt C[<t> <: <t'>](v)  if not (null <: <t'>) and <t> <: <t'>
+```
 
-Coercing a non-null, non-optional and non-reserved value at an option type treats it as an optional value:
+Any other type goes to `null`:
 ```
-C[<t> <: opt <t'>](<v>) = opt C[<t> <: <t'>](v)  if not (null <: <t>) and <t> <: <t'>
-C[<t> <: opt <t'>](_)   = null                   if not (null <: <t>) and not (<t> <: <t'>)
-C[reserved <: opt <t'>](_) = null
+C[reserved <: opt <t>](_) = null
+C[<t> <: opt <t'>](_) = null  if not (null <: <t'>) and not (<t> <: <t'>)
+C[<t> <: opt <t'>](_) = null  if null <: <t'> and not (null <: <t>)
 ```
 
+NOTE: These rules above imply that a Candid decoder has to be able to decide the subtyping relation at runtime.
+
 #### Records
 
 In the following rule: