diff --git a/i386-asm.c b/i386-asm.c
index f9c0fa7..8473d06 100644
--- a/i386-asm.c
+++ b/i386-asm.c
@@ -27,6 +27,7 @@
 
 #define TOK_ASM_first TOK_ASM_clc
 #define TOK_ASM_last TOK_ASM_emms
+#define TOK_ASM_alllast TOK_ASM_pxor
 
 #define OPC_JMP        0x01  /* jmp operand */
 #define OPC_B          0x02  /* only used with OPC_WL */
@@ -715,6 +716,9 @@ ST_FUNC void asm_opcode(TCCState *s1, int opcode)
                 g(b >> 8);
             g(b);
             return;
+        } else if (opcode <= TOK_ASM_alllast) {
+            tcc_error("bad operand with opcode '%s'",
+                  get_tok_str(opcode, NULL));
         } else {
             tcc_error("unknown opcode '%s'",
                   get_tok_str(opcode, NULL));
diff --git a/i386-asm.h b/i386-asm.h
index 760c06d..a954afb 100644
--- a/i386-asm.h
+++ b/i386-asm.h
@@ -463,7 +463,7 @@ ALT(DEF_ASM_OP2(psrlq, 0x0f73, 2, OPC_MODRM, OPT_IM8, OPT_MMX ))
     DEF_ASM_OP2(punpcklbw, 0x0f60, 0, OPC_MODRM, OPT_EA | OPT_MMX, OPT_MMX )
     DEF_ASM_OP2(punpcklwd, 0x0f61, 0, OPC_MODRM, OPT_EA | OPT_MMX, OPT_MMX )
     DEF_ASM_OP2(punpckldq, 0x0f62, 0, OPC_MODRM, OPT_EA | OPT_MMX, OPT_MMX )
-    DEF_ASM_OP2(pxor, 0x0fef, 0, OPC_MODRM, OPT_EA | OPT_MMX, OPT_MMX )
+    DEF_ASM_OP2(pxor, 0x0fef, 0, OPC_MODRM, OPT_EA | OPT_MMX, OPT_MMX ) /* must be last !OP0 */
 
 #undef ALT
 #undef DEF_ASM_OP0
diff --git a/x86_64-asm.h b/x86_64-asm.h
index 69cd77c..31a7b38 100644
--- a/x86_64-asm.h
+++ b/x86_64-asm.h
@@ -438,7 +438,7 @@ ALT(DEF_ASM_OP2(psrlq, 0x0f73, 2, OPC_MODRM, OPT_IM8, OPT_MMX ))
     DEF_ASM_OP2(punpcklbw, 0x0f60, 0, OPC_MODRM, OPT_EA | OPT_MMX, OPT_MMX )
     DEF_ASM_OP2(punpcklwd, 0x0f61, 0, OPC_MODRM, OPT_EA | OPT_MMX, OPT_MMX )
     DEF_ASM_OP2(punpckldq, 0x0f62, 0, OPC_MODRM, OPT_EA | OPT_MMX, OPT_MMX )
-    DEF_ASM_OP2(pxor, 0x0fef, 0, OPC_MODRM, OPT_EA | OPT_MMX, OPT_MMX )
+    DEF_ASM_OP2(pxor, 0x0fef, 0, OPC_MODRM, OPT_EA | OPT_MMX, OPT_MMX ) /* must be last !OP0 */
 
 #undef ALT
 #undef DEF_ASM_OP0