diff --git a/src/vm/stack.cc b/src/vm/stack.cc
index 1a298f0..95908f2 100644
--- a/src/vm/stack.cc
+++ b/src/vm/stack.cc
@@ -16,7 +16,7 @@ void Stack::push(Value const& value)
 
 Value Stack::pop()
 {
-    if (stack_.size() <= fps_.size())
+    if (stack_.size() <= fps_.top())
         throw VMStackUnderflow();
 
     Value v = stack_.back();
@@ -58,4 +58,15 @@ void Stack::pop_fp()
     fps_.pop();
 }
 
+std::string Stack::debug() const
+{
+    if (stack_.empty())
+        return "empty";
+
+    std::string out;
+    for (size_t i = 0; i < stack_.size(); ++i)
+        out += "[" + stack_.at(i).to_string() + "] ";
+    return out;
+}
+
 } // tyche
diff --git a/src/vm/stack.hh b/src/vm/stack.hh
index f42990b..775e902 100644
--- a/src/vm/stack.hh
+++ b/src/vm/stack.hh
@@ -23,6 +23,8 @@ public:
 
     [[nodiscard]] size_t fp_level() const { return fps_.size(); }
 
+    [[nodiscard]] std::string debug() const;
+
 private:
     std::vector<Value> stack_;
     std::stack<size_t> fps_;
diff --git a/src/vm/tests.cc b/src/vm/tests.cc
index 47b29fe..98f809e 100644
--- a/src/vm/tests.cc
+++ b/src/vm/tests.cc
@@ -84,7 +84,9 @@ TEST(VM, BasicCode)
 
     VM vm;
     vm.load_bytecode(std::move(ba));
+    printf("%s\n", vm.debug_stack().c_str());
     vm.call(0);
+    printf("%s\n", vm.debug_stack().c_str());
 
     int32_t result = vm.to_integer(-1);
     ASSERT_EQ(result, 5);
diff --git a/src/vm/value.cc b/src/vm/value.cc
index b051120..40d4f91 100644
--- a/src/vm/value.cc
+++ b/src/vm/value.cc
@@ -15,4 +15,15 @@ Type Value::type() const
     }, value_);
 }
 
+std::string Value::to_string() const
+{
+    return std::visit(overloaded {
+        [](std::monostate)       { return std::string("nil"); },
+        [](int32_t i)            { return std::to_string(i); },
+        [](float f)              { return std::to_string(f); },
+        [](std::string const& s) { return s; },
+        [](Function const& f)    { return "@" + std::to_string(f.f_id); }
+    }, value_);
+}
+
 }
diff --git a/src/vm/value.hh b/src/vm/value.hh
index 4b7ef4a..7aed983 100644
--- a/src/vm/value.hh
+++ b/src/vm/value.hh
@@ -33,6 +33,8 @@ public:
     [[nodiscard]] std::string as_string() const      { return std::get<std::string>(value_); }
     [[nodiscard]] FunctionId  as_function_id() const { return std::get<Function>(value_).f_id; }
 
+    [[nodiscard]] std::string to_string() const;
+
 private:
     using Internal = std::variant<std::monostate, int32_t, float, std::string, Function>;
     Internal value_;
diff --git a/src/vm/vm.hh b/src/vm/vm.hh
index ef8554b..151dc65 100644
--- a/src/vm/vm.hh
+++ b/src/vm/vm.hh
@@ -17,6 +17,8 @@ public:
 
     void push_integer(int32_t value);
 
+    [[nodiscard]] std::string debug_stack() const { return stack_.debug(); }
+
 private:
     void run_until_return();
     void step();