diff --git a/lang/diag.c b/lang/diag.c
index 009fcfd..a95a8bb 100644
--- a/lang/diag.c
+++ b/lang/diag.c
@@ -8,6 +8,7 @@
 
 static const struct ivy_diag_class diag_classes[] = {
 	ERROR_CLASS(IVY_LANG_E_UNRECOGNISED_SYMBOL, "Unrecognised symbol"),
+	ERROR_CLASS(IVY_LANG_E_UNDEFINED_VARIABLE, "Undefined variable"),
 	ERROR_CLASS(
 		IVY_LANG_E_INCORRECT_INLINE_FOR_LOOP,
 		"Incorrect inline for-loop"),
@@ -17,6 +18,8 @@ static const size_t nr_diag_classes = sizeof diag_classes / sizeof diag_classes[
 static const struct ivy_diag_msg diag_msg[] = {
 	MSG(IVY_LANG_MSG_UNKNOWN_SYMBOL_ENCOUNTERED,
             "encountered a symbol that is not part of the Ivy syntax."),
+	MSG(IVY_LANG_MSG_ASSIGN_TO_UNDEFINED_VARIABLE,
+            "cannot use modify-and-assign operator on an undefined variable."),
 	MSG(IVY_LANG_MSG_DO_UNEXPECTED_IN_INLINE_FOR,
             "`[yellow]do[reset]` is not used in inline for-loops."),
 	MSG(IVY_LANG_MSG_PREVIOUS_EXPRESSION_IS_FOR_BODY,
diff --git a/lang/include/ivy/lang/diag.h b/lang/include/ivy/lang/diag.h
index eda974c..e93dc8e 100644
--- a/lang/include/ivy/lang/diag.h
+++ b/lang/include/ivy/lang/diag.h
@@ -8,14 +8,17 @@ struct ivy_diag_ctx;
 enum ivy_lang_diag_code {
 	IVY_LANG_E_NONE = 0,
 	IVY_LANG_E_UNRECOGNISED_SYMBOL,
+	IVY_LANG_E_UNDEFINED_VARIABLE,
 	IVY_LANG_E_INCORRECT_INLINE_FOR_LOOP,
 };
 
 enum ivy_lang_diag_msg {
 	IVY_LANG_MSG_NONE = 0,
+	IVY_LANG_MSG_ASSIGN_TO_UNDEFINED_VARIABLE,
 	IVY_LANG_MSG_UNKNOWN_SYMBOL_ENCOUNTERED,
 	IVY_LANG_MSG_DO_UNEXPECTED_IN_INLINE_FOR,
 	IVY_LANG_MSG_PREVIOUS_EXPRESSION_IS_FOR_BODY,
+	IVY_LANG_MSG_DECLARE_GLOBAL_FOR_ASSIGNMENT,
 };
 
 IVY_API enum ivy_status ivy_lang_diag_ctx_init(struct ivy_diag_ctx *ctx);