diff --git a/uppsrc/CppBase/CppBase.h b/uppsrc/CppBase/CppBase.h
index f9eb60e02..25a77a30b 100644
--- a/uppsrc/CppBase/CppBase.h
+++ b/uppsrc/CppBase/CppBase.h
@@ -104,8 +104,11 @@ class Lex {
 	void Next();
 	bool Prepare(int pos);
 	int  GetCharacter();
+	void ThrowError(const char *e);
 
 public:
+	Callback1<const String&> WhenError;
+
 	struct Grounding {};
 
 	int         Code(int pos = 0);
@@ -392,6 +395,10 @@ class Parser {
 	void   Statement();
 	void   Locals(const String& type);
 	String Tparam(int& q);
+	
+	friend class Lex; // Fix to make Lex::ThrowError
+
+	typedef Parser CLASSNAME;
 
 public:
 	struct FunctionStat
@@ -432,7 +439,7 @@ public:
 	void   Do(Stream& s, const Vector<String>& ignore, CppBase& base, const String& fn,
 	          Callback2<int, const String&> err, const Vector<String>& typenames = Vector<String>());
 
-	Parser() : dobody(false) {}
+	Parser() : dobody(false) { 	lex.WhenError = THISBACK(ThrowError); }
 };
 
 String NoTemplatePars(const String& type);
diff --git a/uppsrc/CppBase/cpplex.cpp b/uppsrc/CppBase/cpplex.cpp
index 3ab3aec81..273341093 100644
--- a/uppsrc/CppBase/cpplex.cpp
+++ b/uppsrc/CppBase/cpplex.cpp
@@ -361,9 +361,16 @@ bool   Lex::IsId(int pos)
 	return Code(pos) >= endkey + 256;
 }
 
+void Lex::ThrowError(const char *e)
+{
+	WhenError(e);
+	throw Parser::Error();
+}
+
 String Lex::Id(int pos)
 {
-	ASSERT(IsId(pos));
+	if(!IsId(pos))
+		ThrowError("expected id");
 	return id[Code(pos) - 256];
 }
 
@@ -414,28 +421,32 @@ void Lex::SkipToGrounding()
 int Lex::Int(int pos)
 {
 	Prepare(pos);
-	ASSERT(term[pos].code == t_integer);
+	if(term[pos].code != t_integer)
+		ThrowError("expected integer literal");
 	return (int)term[pos].number;
 }
 
 double Lex::Double(int pos)
 {
 	Prepare(pos);
-	ASSERT(term[pos].code == t_double);
+	if(term[pos].code != t_double)
+		ThrowError("expected floating point literal");
 	return term[pos].number;
 }
 
 String Lex::Text(int pos)
 {
 	Prepare(pos);
-	ASSERT(term[pos].code == t_string);
+	if(term[pos].code != t_string)
+		ThrowError("expected string literal");
 	return term[pos].text;
 }
 
 int Lex::Chr(int pos)
 {
 	Prepare(pos);
-	ASSERT(term[pos].code == t_character);
+	if(term[pos].code != t_character)
+		ThrowError("expected character literal");
 	return (byte)*term[pos].text;
 }