diff --git a/kalk/src/parser.rs b/kalk/src/parser.rs
index cf126be..26c9303 100644
--- a/kalk/src/parser.rs
+++ b/kalk/src/parser.rs
@@ -112,6 +112,7 @@ pub enum CalcError {
     UndefinedVar(String),
     UnableToInvert(String),
     UnableToSolveEquation,
+    UnableToOverrideConstant(String),
     UnableToParseExpression,
     Unknown,
 }
@@ -140,6 +141,7 @@ impl ToString for CalcError {
             CalcError::UndefinedVar(name) => format!("Undefined variable: '{}'.", name),
             CalcError::UnableToParseExpression => format!("Unable to parse expression."),
             CalcError::UnableToSolveEquation => format!("Unable to solve equation."),
+            CalcError::UnableToOverrideConstant(name) => format!("Unable to override constant: '{}'.", name),
             CalcError::Unknown => format!("Unknown error."),
         }
     }
@@ -317,6 +319,10 @@ fn parse_var_decl_stmt(context: &mut Context) -> Result<Stmt, CalcError> {
         return Err(CalcError::VariableReferencesItself);
     }
 
+    if prelude::is_constant(&identifier.value) {
+        return Err(CalcError::UnableToOverrideConstant(identifier.value.into()));
+    }
+
     Ok(Stmt::VarDecl(
         Identifier::from_full_name(&identifier.value),
         Box::new(expr),
diff --git a/kalk/src/prelude/mod.rs b/kalk/src/prelude/mod.rs
index db2b75c..5271c22 100644
--- a/kalk/src/prelude/mod.rs
+++ b/kalk/src/prelude/mod.rs
@@ -171,6 +171,10 @@ pub fn is_prelude_func(identifier: &str) -> bool {
         || BINARY_FUNCS.contains_key(identifier)
 }
 
+pub fn is_constant(identifier: &str) -> bool {
+    CONSTANTS.contains_key(identifier)
+}
+
 pub fn call_unary_func(
     context: &mut interpreter::Context,
     name: &str,
diff --git a/kalk/src/symbol_table.rs b/kalk/src/symbol_table.rs
index fc78764..207cc1a 100644
--- a/kalk/src/symbol_table.rs
+++ b/kalk/src/symbol_table.rs
@@ -86,7 +86,7 @@ impl SymbolTable {
     }
 
     pub fn contains_var(&self, identifier: &str) -> bool {
-        prelude::CONSTANTS.contains_key(identifier)
+        prelude::is_constant(identifier)
             || identifier == "i"
             || self.hashmap.contains_key(&format!("var.{}", identifier))
     }