diff --git a/interp.c b/interp.c
index a6e7ee5..6aa7db1 100644
--- a/interp.c
+++ b/interp.c
@@ -90,22 +90,26 @@ value_t run_interpreter(value_t lambda, value_t argv)
     {
       /* Builtin functions replace the byte-code and tail-call
        * steps; they also do not require frame variables. */
+      state.nframe = 0;
       _get_builtin_fn(state.lambda.value)(&state);
     }
     else
     {
-      /* Allocate frame variables, initially undefined */
-      state.frame.value = make_vector(get_fixnum(_LAMBDA_SLOT(state.lambda.value, FRAME_VARS)), UNDEFINED);
+      state.nframe = get_fixnum(_LAMBDA_SLOT(state.lambda.value, FRAME_VARS));
+      release_assert((0 <= state.nframe) && (state.nframe <= 120));
 
       run_byte_code(&state);
       perform_tail_call(&state);
     }
 
-    /* Clear temporaries so they can be GC'd. */
-    state.frame.value = UNDEFINED;
-    state.in1.value   = UNDEFINED;
-    state.in2.value   = UNDEFINED;
-    state.in3.value   = UNDEFINED;
+    /* Clear (used) frame-variable slots so they can be GC'd. */
+    for (fixnum_t i = 0; i < state.nframe; ++i)
+      _get_vector(state.frame.value)->elements[i] = UNDEFINED;
+
+    /* Clear temporaries. */
+    state.in1.value = UNDEFINED;
+    state.in2.value = UNDEFINED;
+    state.in3.value = UNDEFINED;
 
     if (run_finalizers)
     {
@@ -504,7 +508,7 @@ static value_t get_input(const interp_state_t *state, fixnum_t var)
       vector_t *vec = _get_vector(state->frame.value);
       var -= 128;
 
-      release_assert(var < vec->size);
+      release_assert(var < state->nframe);
       return vec->elements[var];
     }
   /* 248 ... 252 are reserved */
@@ -527,15 +531,15 @@ static void set_output(const interp_state_t *state, fixnum_t var, value_t val)
   release_assert((var >= 128) && (var <= 255));
 
   var -= 128;
-  release_assert(var < vec->size);
+  release_assert(var < state->nframe);
   vec->elements[var] = val;
 }
 
 static void register_state(interp_state_t *state, value_t lambda, value_t argv)
 {
   register_gc_root(&state->lambda, lambda);
-  register_gc_root(&state->frame,  UNDEFINED);
   register_gc_root(&state->argv,   argv);
+  register_gc_root(&state->frame,  make_vector(120, UNDEFINED));
   register_gc_root(&state->k,      FALSE_VALUE);
   register_gc_root(&state->ctx,    FALSE_VALUE);
   register_gc_root(&state->in1,    FALSE_VALUE);
@@ -546,8 +550,8 @@ static void register_state(interp_state_t *state, value_t lambda, value_t argv)
 static void unregister_state(interp_state_t *state)
 {
   unregister_gc_root(&state->lambda);
-  unregister_gc_root(&state->frame);
   unregister_gc_root(&state->argv);
+  unregister_gc_root(&state->frame);
   unregister_gc_root(&state->k);
   unregister_gc_root(&state->ctx);
   unregister_gc_root(&state->in1);
diff --git a/interp.h b/interp.h
index 5cf6bd2..6f46889 100644
--- a/interp.h
+++ b/interp.h
@@ -1,9 +1,8 @@
-/* Need to ensure gc.h is included first, because it depends on interp.h. */
-#include "gc.h"
-
 #ifndef INTERP_H_9c7eea5c5cd0f7a32b79a8ca0ab2969f
 #define INTERP_H_9c7eea5c5cd0f7a32b79a8ca0ab2969f
 
+#include "gc.h"
+
 typedef struct interp_state
 {
   gc_root_t lambda;
@@ -14,6 +13,7 @@ typedef struct interp_state
   gc_root_t in1;
   gc_root_t in2;
   gc_root_t in3;
+  fixnum_t nframe;
 } interp_state_t;
 
 void interpreter_init(void);