diff --git a/spec/errors.md b/spec/errors.md
index 5e49c1cd38..71f72ec3fd 100644
--- a/spec/errors.md
+++ b/spec/errors.md
@@ -238,6 +238,34 @@ or for private implementation use that is not supported by the current implement
 > * {{The value is not one.}}
 > ```
 
+### Invalid Expression
+
+An **_<dfn>Invalid Expression</dfn>_** error occurs when a _message_ includes an _expression_
+whose implementation-defined internal requirements produce an error during _function resolution_
+or when a _function_ returns a value (such as `null`) that the implementation does not support.
+
+An **_<dfn>Operand Mismatch Error</dfn>_** is an _Invalid Expression_ error that occurs when
+an _operand_ provided to a _function_ during _function resolution_ does not match one of the
+expected implementation-defined types for that function;
+or in which a literal _operand_ value does not have the required format
+and thus cannot be processed into one of the expected implementation-defined types
+for that specific _function_.
+
+> For example, the following _message_ produces an _Operand Mismatch Error_
+> (a type of _Invalid Expression_ error)
+> because the literal `|horse|` does not match the production `number-literal`,
+> which is a requirement of the function `:number` for its operand:
+> ```
+> .local $horse = {horse :number}
+> {{You have a {$horse}.}}
+> ```
+> The following _message_ might produce an _Invalid Expression_ error if the
+> the function `:function` threw an exception or otherwise emitted an error
+> rather than returning a valid value:
+>```
+> {{This has an invalid expression {$var :function} because it has a bug in it.}}
+>```
+
 ### Unsupported Statement
 
 An **_<dfn>Unsupported Statement</dfn>_** error occurs when a message includes a _reserved statement_.
diff --git a/spec/formatting.md b/spec/formatting.md
index dd71c96a0f..2379301cbb 100644
--- a/spec/formatting.md
+++ b/spec/formatting.md
@@ -237,8 +237,17 @@ the following steps are taken:
 
 5. If the call succeeds,
    resolve the value of the _expression_ as the result of that function call.
+
    If the call fails or does not return a valid value,
-   emit a _Resolution Error_ and use a _fallback value_ for the _expression_.
+   emit a _Invalid Expression_ error.
+
+   Implementations MAY provide a mechanism for the _function_ to provide
+   additional detail about internal failures.
+   Specifically, if the cause of the failure was that the datatype, value, or format of the
+   _operand_ did not match that expected by the _function_,
+   the _function_ might cause an _Operand Mismatch Error_ to be emitted.
+  
+   In all failure cases, use the _fallback value_ for the _expression_ as the resolved value.
 
 #### Option Resolution