diff --git a/src/abstract_tree/as.rs b/src/abstract_tree/as.rs
index 210bc37..3e87b9d 100644
--- a/src/abstract_tree/as.rs
+++ b/src/abstract_tree/as.rs
@@ -57,25 +57,18 @@ impl AbstractTree for As {
                         });
                     }
                 }
+
                 Type::Any => {
                     // Do no validation when converting from "any" to a list.
                     // This effectively defers to runtime behavior, potentially
                     // causing a runtime error.
                 }
-                Type::Boolean => todo!(),
-                Type::Collection => todo!(),
-                Type::Custom(_) => todo!(),
-                Type::Float => todo!(),
-                Type::Function {
-                    parameter_types: _,
-                    return_type: _,
-                } => todo!(),
-                Type::Integer => todo!(),
-                Type::Map(_) => todo!(),
-                Type::None => todo!(),
-                Type::Number => todo!(),
-                Type::Range => todo!(),
-                Type::Option(_) => todo!(),
+                _ => {
+                    return Err(ValidationError::ConversionImpossible {
+                        initial_type,
+                        target_type: self.r#type.clone(),
+                    })
+                }
             }
         }
 
diff --git a/src/abstract_tree/function_call.rs b/src/abstract_tree/function_call.rs
index 621ff2e..6d9d189 100644
--- a/src/abstract_tree/function_call.rs
+++ b/src/abstract_tree/function_call.rs
@@ -87,7 +87,15 @@ impl AbstractTree for FunctionCall {
                     Ok(value_type)
                 }
             }
-            FunctionExpression::Index(index) => index.expected_type(context),
+            FunctionExpression::Index(index) => {
+                let index_type = index.expected_type(context)?;
+
+                if let Type::Function { return_type, .. } = index_type {
+                    Ok(*return_type)
+                } else {
+                    Ok(index_type)
+                }
+            }
         }
     }
 
diff --git a/tests/as.rs b/tests/as.rs
index 61add06..1be8996 100644
--- a/tests/as.rs
+++ b/tests/as.rs
@@ -39,7 +39,7 @@ fn conversion_runtime_error() {
         interpret(&format!("json:parse('{JSON}') as [map]")),
         Err(Error::Runtime(RuntimeError::ConversionImpossible {
             value: json_value,
-            target_type: Type::List(Box::new(Type::Float))
+            target_type: Type::List(Box::new(Type::Map(None)))
         }))
     )
 }