diff --git a/include/unicorn/unicorn.h b/include/unicorn/unicorn.h
index 161e7613..a3293920 100644
--- a/include/unicorn/unicorn.h
+++ b/include/unicorn/unicorn.h
@@ -121,7 +121,7 @@ typedef enum uc_err {
 
 // Callback function for tracing code (UC_HOOK_CODE & UC_HOOK_BLOCK)
 // @address: address where the code is being executed
-// @size: size of machine instruction being executed
+// @size: size of machine instruction(s) being executed, or 0 when size is unknown
 // @user_data: user data passed to tracing APIs.
 typedef void (*uc_cb_hookcode_t)(uch handle, uint64_t address, uint32_t size, void *user_data);
 
diff --git a/qemu/translate-all.c b/qemu/translate-all.c
index 9723f23f..aaebee42 100644
--- a/qemu/translate-all.c
+++ b/qemu/translate-all.c
@@ -179,8 +179,10 @@ static int cpu_gen_code(CPUArchState *env, TranslationBlock *tb, int *gen_code_s
     gen_intermediate_code(env, tb);
 
     // Unicorn: when tracing block, patch 1st operand for block size
-    if (env->uc->hook_block) {
-        if (env->uc->block_addr == tb->pc)
+    if (env->uc->hook_block && env->uc->block_addr == tb->pc) {
+        if (env->uc->block_full)    // block size is unknown
+            *(s->gen_opparam_buf + 1) = 0;
+        else
             *(s->gen_opparam_buf + 1) = tb->size;
     }