diff --git a/src/assembler.cpp b/src/assembler.cpp
index d0fa34e..96ed892 100644
--- a/src/assembler.cpp
+++ b/src/assembler.cpp
@@ -49,8 +49,11 @@ Number to_number(std::string_view txt, Base base, int skip = 0)
     return static_cast<Number>(result);
 }
 
-std::string any_to_string(std::any const& val)
+std::string any_to_string(std::any const& val, std::string_view name)
 {
+    if (!val.has_value()) {
+        throw parse_error(fmt::format("Cannot convert 'None' value from Symbol {} to std::string", name));
+    }
     if (auto const* n = std::any_cast<Number>(&val)) {
         auto in = static_cast<int64_t>(*n);
         if (*n == static_cast<double>(in)) return fmt::format("${:x}", in);
@@ -547,8 +550,11 @@ void Assembler::setSym(std::string_view sym, std::any val)
     syms.set_final(sym);
 }
 
-AsmValue to_variant(std::any const& a)
+AsmValue to_variant(std::any const& a, std::string_view identifier)
 {
+    if (!a.has_value()) {
+        throw parse_error(fmt::format("Cannot convert a 'None' {} value to AsmValue", identifier));
+    }
     if (auto const* n = std::any_cast<Number>(&a))
     {
         return *n;
@@ -565,7 +571,12 @@ AsmValue to_variant(std::any const& a)
     {
         return *vn;
     }
-    return {};
+
+    // not implemented
+    static_assert(std::variant_size<AsmValue>() == 4);
+    //throw parse_error(fmt::format("No variant conversion available for type {}", a.type().name()));
+    auto s_err = fmt::format("No AsmValue conversion available for type {} (context: {})", a.type().name(), identifier);
+    return persist(s_err);
 }
 
 void Assembler::setupRules()
@@ -732,8 +743,21 @@ void Assembler::setupRules()
         }
 
         for(auto&& a : meta.args) {
-            auto v = to_variant(a);
-            meta.vargs.push_back(v);
+            auto identifier = fmt::format("MetaBlock::{} -> arg[{}]", meta.name, meta.vargs.size());
+            if (a.has_value()) {
+                auto v = to_variant(a, identifier);
+                meta.vargs.push_back(v);
+                if (auto strv = std::get_if<std::string_view>(&v)) {
+                    if (strv->starts_with("No AsmValue conversion")) {
+                        LOGD(fmt::format("{}\n", *strv));
+                    }
+                }
+            } else {
+                // FIXME: keeps existing code intact (can we leave out such values?)
+                AsmValue v_default;
+                LOGD(fmt::format("AsmValue cannot hold a 'None' value -> creating value {} (context: {}))", std::get<0>(v_default), identifier));
+                meta.vargs.push_back(v_default);
+            }
         }
 
 
@@ -969,7 +993,7 @@ void Assembler::setupRules()
     auto buildArg = [](SV& sv) -> std::any {
         auto mode = modeMap.at(std::string(sv.name()));
         return Instruction{"", mode,
-                           mode == Mode::ACC ? 0 : any_cast<Number>(sv[0])};
+                           mode == Mode::ACC ? 0 : number<int32_t>(sv[0])};
     };
     for (auto const& [name, _] : modeMap) {
         parser.after(name.c_str(), buildArg);
@@ -978,7 +1002,7 @@ void Assembler::setupRules()
     parser.after("ZRel", [](SV& sv) -> Instruction {
         auto v = (number<int32_t>(sv[1]) << 24) |
                  (number<int32_t>(sv[0]) << 16) | number<int32_t>(sv[2]);
-        return {"", Mode::ZP_REL, static_cast<Number>(v)};
+        return {"", Mode::ZP_REL, v};
     });
 
     parser.after("LabelRef", [this](SV& sv) {
@@ -1099,8 +1123,8 @@ void Assembler::setupRules()
         }
 
         auto ope = any_cast<std::string_view>(sv[1]);
-        auto arg1 = to_variant(sv[0]);
-        auto arg2 = to_variant(sv[2]);
+        auto arg1 = to_variant(sv[0], fmt::format("Expression2 -> arg[{}]", 0));
+        auto arg2 = to_variant(sv[2], fmt::format("Expression2 -> arg[{}]", 2));
 
       try {
           auto res = operation(ope, arg1, arg2);
@@ -1400,7 +1424,7 @@ void Assembler::printSymbols()
 {
     syms.forAll([](std::string const& name, std::any const& val) {
         if (!utils::startsWith(name, "__"))
-            fmt::print("{} == {}\n", name, any_to_string(val));
+            fmt::print("{} == {}\n", name, any_to_string(val, name));
     });
 }
 
@@ -1411,7 +1435,7 @@ void Assembler::writeSymbols(fs::path const& p)
         if (!utils::startsWith(name, "__") &&
             (name.find('.') == std::string::npos) &&
             val.type() == typeid(Number))
-            fmt::print(f.filePointer(), "{} = {}\n", name, any_to_string(val));
+            fmt::print(f.filePointer(), "{} = {}\n", name, any_to_string(val, name));
     });
 }
 
diff --git a/src/assembler.h b/src/assembler.h
index 4c7ca5c..c515fa9 100644
--- a/src/assembler.h
+++ b/src/assembler.h
@@ -14,8 +14,6 @@
 
 class Machine;
 
-using AsmValue = std::variant<Number, std::string_view, std::vector<uint8_t>, std::vector<Number>>;
-
 inline void Check(bool v, std::string const& txt)
 {
     if (!v) throw parse_error(txt);
diff --git a/src/defines.h b/src/defines.h
index 2d7cf62..cd3096b 100644
--- a/src/defines.h
+++ b/src/defines.h
@@ -9,6 +9,7 @@
 #include <string>
 #include <unordered_map>
 #include <unordered_set>
+#include <variant>
 #include <vector>
 
 #include <filesystem>
@@ -110,6 +111,7 @@ inline std::string_view operator+(std::string_view sv, std::string_view n)
 }
 
 using Number = double;
+using AsmValue = std::variant<Number, std::string_view, std::vector<uint8_t>, std::vector<Number>>;
 
 inline Num div(Num a, Num b)
 {
@@ -125,12 +127,18 @@ inline Num pow(Num a, Num b)
 template <typename T>
 inline T number(std::any const& v)
 {
-    return static_cast<T>(std::any_cast<Number>(v));
+    if (v.has_value()) {
+        return static_cast<T>(std::any_cast<Number>(v));
+    }
+    throw std::runtime_error(fmt::format("Cannot convert 'None' to arithmetic type '{}'.", typeid(T).name()));
 }
 
 inline Number number(std::any const& v)
 {
-    return std::any_cast<Number>(v);
+    if (v.has_value()) {
+        return std::any_cast<Number>(v);
+    }
+    throw std::runtime_error(fmt::format("Cannot converted 'None' to Number type ({})", typeid(Number).name()));
 }
 
 template <typename T>
@@ -158,8 +166,8 @@ class assert_error : public std::exception
 
 struct Instruction
 {
-    Instruction(std::string_view op, sixfive::Mode m, double v)
-        : opcode(op), mode(m), val(static_cast<int32_t>(v))
+    Instruction(std::string_view op, sixfive::Mode m, int32_t v)
+        : opcode(op), mode(m), val(v)
     {}
     std::string opcode;
     sixfive::Mode mode;
@@ -190,7 +198,7 @@ inline std::string getHomeDir()
     return homeDir;
 }
 
-std::string any_to_string(std::any const& val);
+std::string any_to_string(std::any const& val, std::string_view name);
 
 inline void printArg(std::any const& arg)
 {
diff --git a/src/machine.cpp b/src/machine.cpp
index 9a6e53a..0274181 100644
--- a/src/machine.cpp
+++ b/src/machine.cpp
@@ -742,7 +742,7 @@ AsmResult Machine::assemble(Instruction const& instr)
     // Find a matching addressing mode
     auto it_op =
         std::find_if(it_ins->opcodes.begin(), it_ins->opcodes.end(),
-                     [&](auto const& o) { 
+                     [&](auto const& o) {
                         return modeMatches(arg.mode, o.mode, small);
                      });
 
diff --git a/src/parser.cpp b/src/parser.cpp
index cd96d0b..bdec4b6 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -263,10 +263,10 @@ std::any Parser::evaluate(AstNode const& node)
                            sv.name(), ast->line, sv.token_view());
                 for (size_t i = 0; i < sv.size(); i++) {
                     std::any const v = sv[i];
-                    fmt::print("  {}: {}\n", i, any_to_string(v));
+                    fmt::print("  {}: {}\n", i, any_to_string(v, sv.name()));
                 }
                 auto ret = callAction(sv, *ast->action);
-                fmt::print(">>  {}\n", any_to_string(ret));
+                fmt::print(">>  {}\n", any_to_string(ret, sv.name()));
                 return ret;
             }
             return callAction(sv, *ast->action);
diff --git a/src/parser.h b/src/parser.h
index 7e79960..e3c3abe 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -67,12 +67,6 @@ class SemanticValues
 
     AstNode get_node() const { return ast; }
     std::string const& file_name() const;
-
-    template <typename T>
-    T to(size_t i) const
-    {
-        return std::any_cast<T>(operator[](i));
-    }
 };
 
 using ActionFn = std::function<std::any(SemanticValues const&)>;
diff --git a/src/symbol_table.h b/src/symbol_table.h
index 486e687..5bf7626 100644
--- a/src/symbol_table.h
+++ b/src/symbol_table.h
@@ -258,6 +258,7 @@ struct SymbolTable
             }
             undefined.insert(std::string{name});
             if constexpr (std::is_same_v<T, std::any>) {
+                LOGD(fmt::format("Returning zero value for undefined label '{}'\n", name));
                 return zero;
             }
             LOGD("Returning default (%s)", typeid(T{}).name());