throw error with non-register operands

twasm/asm/main.asm

@@ -661,17 +661,27 @@ tokenise:
 
     mov cx, ax ; cx = length counter for safe keeping
     call evaluate_operand
+    ; dl = return code
     ; ax = register's token ID
-
     pop rsi
     pop rdi      ; rdi = first byte of operand
-    add di, cx ; rdi = last byte of operand
-    mov cx, ax ; cx = register's token ID for safe keeping
+    add di, cx   ; rdi = last byte of operand
+    mov rcx, rax ; rcx = evaluate_operand's return value
     pop rax      ; rax = number of tokens processed
 
+    ; operand is a register
+    ; cx = token ID
+    cmp dl, 0x00
+    je .operand_register
+
+    jmp .unexpected_operand
+
+  .operand_register:
     mov [TOKEN_TABLE_ADDR + rax * TOKEN_TABLE_ENTRY_SIZE], cx
     inc rax ; another token processed
+    jmp .operand_break_continue
 
+  .operand_break_continue:
     mov byte [.expecting], E_COMMENT | E_NEWLINE | E_WHITESPACE | E_COMMA
     jmp .loop