diff --git a/src/abstract_tree/as.rs b/src/abstract_tree/as.rs
index 444722c..210bc37 100644
--- a/src/abstract_tree/as.rs
+++ b/src/abstract_tree/as.rs
@@ -57,7 +57,11 @@ impl AbstractTree for As {
                         });
                     }
                 }
-                Type::Any => todo!(),
+                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!(),
@@ -91,14 +95,12 @@ impl AbstractTree for As {
 
                     Value::List(List::with_items(chars))
                 }
-                Value::Map(_) => todo!(),
-                Value::Function(_) => todo!(),
-                Value::Float(_) => todo!(),
-                Value::Integer(_) => todo!(),
-                Value::Boolean(_) => todo!(),
-                Value::Range(_) => todo!(),
-                Value::Option(_) => todo!(),
-                Value::Structure(_) => todo!(),
+                _ => {
+                    return Err(RuntimeError::ConversionImpossible {
+                        value,
+                        target_type: self.r#type.clone(),
+                    });
+                }
             }
         } else {
             todo!()
diff --git a/src/abstract_tree/built_in_value.rs b/src/abstract_tree/built_in_value.rs
index 5bc4c35..a71114f 100644
--- a/src/abstract_tree/built_in_value.rs
+++ b/src/abstract_tree/built_in_value.rs
@@ -6,7 +6,8 @@ use serde::{Deserialize, Serialize};
 use crate::{
     built_in_functions::{fs::fs_functions, json::json_functions, str::string_functions, Callable},
     error::{RuntimeError, SyntaxError, ValidationError},
-    AbstractTree, BuiltInFunction, Context, Format, Function, List, Map, SyntaxNode, Type, Value,
+    AbstractTree, BuiltInFunction, Context, Format, Function, List, Map, Structure, SyntaxNode,
+    Type, Value,
 };
 
 static ARGS: OnceLock<Value> = OnceLock::new();
@@ -87,7 +88,9 @@ impl BuiltInValue {
             BuiltInValue::Args => Type::list(Type::String),
             BuiltInValue::AssertEqual => BuiltInFunction::AssertEqual.r#type(),
             BuiltInValue::Fs => Type::Map(None),
-            BuiltInValue::Json => Type::Map(None),
+            BuiltInValue::Json => Type::Map(Some(
+                Structure::from_map(self.get().as_map().unwrap()).unwrap(),
+            )),
             BuiltInValue::Length => BuiltInFunction::Length.r#type(),
             BuiltInValue::Output => BuiltInFunction::Output.r#type(),
             BuiltInValue::Random => Type::Map(None),
diff --git a/src/abstract_tree/index.rs b/src/abstract_tree/index.rs
index 853d153..a401235 100644
--- a/src/abstract_tree/index.rs
+++ b/src/abstract_tree/index.rs
@@ -48,7 +48,17 @@ impl AbstractTree for Index {
     fn expected_type(&self, context: &Context) -> Result<Type, ValidationError> {
         match self.collection.expected_type(context)? {
             Type::List(item_type) => Ok(*item_type.clone()),
-            Type::Map(_) => Ok(Type::Any),
+            Type::Map(structure) => {
+                if let Some(structure) = structure {
+                    if let IndexExpression::Identifier(identifier) = &self.index {
+                        if let Some((_, r#type)) = structure.inner().get(identifier.inner()) {
+                            return Ok(r#type.clone());
+                        }
+                    }
+                }
+
+                Ok(Type::Any)
+            }
             Type::None => Ok(Type::None),
             r#type => Ok(r#type),
         }
diff --git a/src/error/runtime_error.rs b/src/error/runtime_error.rs
index 68eb221..94ff109 100644
--- a/src/error/runtime_error.rs
+++ b/src/error/runtime_error.rs
@@ -7,12 +7,18 @@ use std::{
     time,
 };
 
-use crate::Value;
+use crate::{Type, Value};
 
 use super::rw_lock_error::RwLockError;
 
 #[derive(Debug, PartialEq)]
 pub enum RuntimeError {
+    /// The attempted conversion is impossible.
+    ConversionImpossible {
+        value: Value,
+        target_type: Type,
+    },
+
     Csv(String),
 
     Io(String),
diff --git a/src/value/structure.rs b/src/value/structure.rs
index 52bb835..0f13804 100644
--- a/src/value/structure.rs
+++ b/src/value/structure.rs
@@ -11,7 +11,7 @@ use serde::{
     Deserialize, Deserializer, Serialize, Serializer,
 };
 
-use crate::{Type, Value};
+use crate::{error::rw_lock_error::RwLockError, Map, Type, Value};
 
 #[derive(Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
 pub struct Structure(Arc<BTreeMap<String, (Option<Value>, Type)>>);
@@ -21,6 +21,16 @@ impl Structure {
         Structure(Arc::new(map))
     }
 
+    pub fn from_map(map: &Map) -> Result<Self, RwLockError> {
+        let mut structure = BTreeMap::new();
+
+        for (key, value) in map.inner()?.iter() {
+            structure.insert(key.clone(), (Some(value.clone()), value.r#type()));
+        }
+
+        Ok(Structure(Arc::new(structure)))
+    }
+
     pub fn inner(&self) -> &BTreeMap<String, (Option<Value>, Type)> {
         &self.0
     }
@@ -42,7 +52,7 @@ impl Display for Structure {
 }
 
 impl Serialize for Structure {
-    fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
+    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where
         S: Serializer,
     {
@@ -94,7 +104,7 @@ impl<'de> Visitor<'de> for StructureVisitor {
 }
 
 impl<'de> Deserialize<'de> for Structure {
-    fn deserialize<D>(deserializer: D) -> std::result::Result<Self, D::Error>
+    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
         D: Deserializer<'de>,
     {
diff --git a/tests/as.rs b/tests/as.rs
index 2a3cc0d..61add06 100644
--- a/tests/as.rs
+++ b/tests/as.rs
@@ -1,4 +1,7 @@
-use dust_lang::{error::ValidationError, *};
+use dust_lang::{
+    error::{RuntimeError, ValidationError},
+    *,
+};
 
 #[test]
 fn string_as_list() {
@@ -25,3 +28,18 @@ fn string_as_list_conversion_error() {
         }))
     )
 }
+
+const JSON: &str = "{ \"x\": 1 }";
+
+#[test]
+fn conversion_runtime_error() {
+    let json_value = interpret(&format!("json:parse('{JSON}')")).unwrap();
+
+    assert_eq!(
+        interpret(&format!("json:parse('{JSON}') as [map]")),
+        Err(Error::Runtime(RuntimeError::ConversionImpossible {
+            value: json_value,
+            target_type: Type::List(Box::new(Type::Float))
+        }))
+    )
+}