Mention that variable values need to be coerced for execution.

spec/Section 3 -- Type System.md

@@ -735,11 +735,11 @@ query withNullableVariable($var: String) {
 Hence, if the value for a Non Null type is hard-coded in the query, it is always
 coerced using the input coercion for the wrapped type.
 
-When a Non Null input has its value set using a variable, the coerced value
-should be `null` if the provided value is `null`-like in the provided
-representation, or if the provided value is omitted. Otherwise, the coerced
-value is the result of running the wrapped type's input coercion on the
-provided value.
+When a Non Null input has its value set using a variable, a query
+error must be raised if the provided value is `null`-like in the
+provided representation, or if the provided value is omitted.
+Otherwise, the coerced value is the result of running the wrapped
+type's input coercion on the provided value.
 
 ## Directives
 

spec/Section 6 -- Execution.md

@@ -14,6 +14,14 @@ error. If the operation is found, then the result of evaluating the request
 should be the result of evaluating the operation according to the “Evaluating
 operations” section.
 
+## Coercing Variables
+
+If the operation has defined any variables, then the values for
+those variables need to be coerced using the input coercion rules
+of variable's declared type. If a query error is encountered during
+input coercion of variable values, then the operation fails without
+execution.
+
 ## Evaluating operations
 
 The type system, as described in the “Type System” part of the spec, must