diff --git a/src/include/interpreter.h b/src/include/interpreter.h
index e4b5a64..02b895d 100644
--- a/src/include/interpreter.h
+++ b/src/include/interpreter.h
@@ -2,10 +2,18 @@
 #define INTERPRETER_H
 
 #include <variant>
+#include <string>
+#include <stdexcept>
 using namespace std;
 
 using EvalResult = variant<int, monostate>;
 
+class RuntimeError : public runtime_error {
+public:
+    explicit RuntimeError(const string& message)
+        : runtime_error(message) {}
+};
+
 /*
     Evaluates the AST, returning the latest calulated value
 */
diff --git a/src/include/tokenize.h b/src/include/tokenize.h
index 167b490..6310ab3 100644
--- a/src/include/tokenize.h
+++ b/src/include/tokenize.h
@@ -17,10 +17,10 @@ struct Token {
     TokenData data { };
 };
 
-class TokenError : public std::runtime_error {
+class TokenError : public runtime_error {
 public:
-    explicit TokenError(const std::string& message)
-        : std::runtime_error(message) {}
+    explicit TokenError(const string& message)
+        : runtime_error(message) {}
 };
 
 /*
diff --git a/src/interpreter.cpp b/src/interpreter.cpp
index 7b246a5..55ce148 100644
--- a/src/interpreter.cpp
+++ b/src/interpreter.cpp
@@ -37,13 +37,13 @@ EvalResult eval(Node &ast) {
             case NodeType::Div: {
                 int e1 = get<int>(eval(node.children[0]));
                 int e2 = get<int>(eval(node.children[1]));
-                // if (e2 == 0) 
+                if (e2 == 0) throw RuntimeError("Division by 0");
                 return e1 / e2;
             } break;
             case NodeType::Mod: {
                 int e1 = get<int>(eval(node.children[0]));
                 int e2 = get<int>(eval(node.children[1]));
-                // if (e2 == 0)
+                if (e2 == 0) throw RuntimeError("Modulo by 0");
                 return e1 % e2;
             } break;
             case NodeType::UnaryPlus: {
@@ -106,7 +106,7 @@ EvalResult eval(Node &ast) {
             } break;
             case TokenType::Identifier: {
                 string identifier = get<string>(token.data);
-                // if (!memory.contains(identifier)) return;
+                if (!memory.contains(identifier)) throw RuntimeError("Unknown identifier");
                 return memory[identifier];
             } break;
             default: