diff --git a/doc/bytecode.txt b/doc/bytecode.txt
index 7ac5b99..e778127 100644
--- a/doc/bytecode.txt
+++ b/doc/bytecode.txt
@@ -12,6 +12,7 @@ unary-expr: up to 255, 1 out, 1 in
   02 (set! out (unbox in))
   03 (set! out (car   in))
   04 (set! out (cdr   in))
+  05 (set! out (weak-unbox in))
 
   08 (set! out (boolean?      in))  ; value => bool
   09 (set! out (fixnum?       in))  ; value => bool
diff --git a/interp.c b/interp.c
index 7d962b1..81a9a89 100644
--- a/interp.c
+++ b/interp.c
@@ -407,9 +407,10 @@ static value_t eval_unary_expression(interp_state_t *state, uint8_t subcode, uin
   switch (subcode)
   {
   case 0x01: return ST1;
-  case 0x02: return is_weak_box(ST1) ? _get_weak_box(ST1)->value : get_box(ST1)->value;
+  case 0x02: return get_box(ST1)->value;
   case 0x03: return get_pair(ST1)->car;
   case 0x04: return get_pair(ST1)->cdr;
+  case 0x05: return get_weak_box(ST1)->value;
   case 0x08: return boolean_value(is_boolean(ST1));
   case 0x09: return boolean_value(is_fixnum(ST1));
   case 0x0a: return boolean_value(is_box(ST1));
diff --git a/libcompiler/primitives.scm b/libcompiler/primitives.scm
index d293b0b..a5e34a4 100644
--- a/libcompiler/primitives.scm
+++ b/libcompiler/primitives.scm
@@ -23,6 +23,7 @@
   '((#%unbox            #x02 unbox)
     (#%car              #x03 car)
     (#%cdr              #x04 cdr)
+    (#%weak-unbox       #x05 weak-unbox)
     (#%boolean?         #x08 boolean?)
     (#%fixnum?          #x09 fixnum?)
     (#%box?             #x0a box?)
diff --git a/src/lib/primitives.rls b/src/lib/primitives.rls
index 4f451b6..430003a 100644
--- a/src/lib/primitives.rls
+++ b/src/lib/primitives.rls
@@ -4,6 +4,7 @@
 (define (unbox x) (unbox x))
 (define (car x) (car x))
 (define (cdr x) (cdr x))
+(define (weak-unbox x) (weak-unbox x))
 (define (boolean? x) (boolean? x))
 (define (fixnum? x) (fixnum? x))
 (define (box? x) (box? x))