diff --git a/src/interpreter.cpp b/src/interpreter.cpp
index c42448d..acbc30d 100644
--- a/src/interpreter.cpp
+++ b/src/interpreter.cpp
@@ -285,6 +285,9 @@ EvalResult eval(Node &ast, Memory &memory) {
                 string typeName = get<string>(typeTok.data);
                 string identifier = get<string>(identifierTok.data);
 
+                if (memory.contains(identifier))
+                    throw RuntimeError("Already defined identifier \""+identifier+"\"", identifierTok.pos);
+
                 memory.declare(identifier, string_to_type(typeName));
                 
                 return {};
@@ -296,6 +299,9 @@ EvalResult eval(Node &ast, Memory &memory) {
                 string identifier = get<string>(identifierTok.data);
                 EvalResult value = eval(node.children[2], memory);
 
+                if (memory.contains(identifier))
+                    throw RuntimeError("Already defined identifier \""+identifier+"\"", identifierTok.pos);
+
                 Type type = string_to_type(typeName);
                 memory.declare(identifier, type);
                 
@@ -313,6 +319,9 @@ EvalResult eval(Node &ast, Memory &memory) {
                 string identifier = get<string>(identifierTok.data);
                 EvalResult value = eval(node.children[1], memory);
 
+                if (!memory.contains(identifier))
+                    throw RuntimeError("Unknown identifier \""+identifier+"\"", identifierTok.pos);
+
                 Type type = memory.get(identifier).type;
 
                 if (type == Type::Int) {
@@ -328,6 +337,9 @@ EvalResult eval(Node &ast, Memory &memory) {
                 Token identifierTok = get<Token>(node.children[0]);
                 string identifier = get<string>(identifierTok.data);
 
+                if (!memory.contains(identifier))
+                    throw RuntimeError("Unknown identifier \""+identifier+"\"", identifierTok.pos);
+
                 MemoryVar& var = memory.get(identifier);
 
                 if (!var.initialized) {
@@ -345,6 +357,9 @@ EvalResult eval(Node &ast, Memory &memory) {
                 Token identifierTok = get<Token>(node.children[0]);
                 string identifier = get<string>(identifierTok.data);
 
+                if (!memory.contains(identifier))
+                    throw RuntimeError("Unknown identifier \""+identifier+"\"", identifierTok.pos);
+
                 MemoryVar& var = memory.get(identifier);
 
                 if (!var.initialized) {
@@ -363,6 +378,9 @@ EvalResult eval(Node &ast, Memory &memory) {
                 Token identifierTok = get<Token>(node.children[0]);
                 string identifier = get<string>(identifierTok.data);
 
+                if (!memory.contains(identifier))
+                    throw RuntimeError("Unknown identifier \""+identifier+"\"", identifierTok.pos);
+
                 MemoryVar& var = memory.get(identifier);
 
                 if (!var.initialized) {
@@ -380,6 +398,9 @@ EvalResult eval(Node &ast, Memory &memory) {
                 Token identifierTok = get<Token>(node.children[0]);
                 string identifier = get<string>(identifierTok.data);
 
+                if (!memory.contains(identifier))
+                    throw RuntimeError("Unknown identifier \""+identifier+"\"", identifierTok.pos);
+
                 MemoryVar& var = memory.get(identifier);
 
                 if (!var.initialized) {
@@ -414,10 +435,14 @@ EvalResult eval(Node &ast, Memory &memory) {
             } break;
             case TokenType::Identifier: {
                 string identifier = get<string>(token.data);
+
+                if (!memory.contains(identifier))
+                    throw RuntimeError("Unknown identifier \""+identifier+"\"", token.pos);
                 
                 MemoryVar& var = memory.get(identifier);
 
-                if (!var.initialized) throw RuntimeError("Accessing uninitialized identifier \""+identifier+"\"", token.pos);
+                if (!var.initialized)
+                    throw RuntimeError("Accessing uninitialized identifier \""+identifier+"\"", token.pos);
                 
                 return var.value;
             } break;