diff --git a/libguile/whippet/api/mmc-attrs.h b/libguile/whippet/api/mmc-attrs.h
index 116c64036..a9fdcc844 100644
--- a/libguile/whippet/api/mmc-attrs.h
+++ b/libguile/whippet/api/mmc-attrs.h
@@ -35,6 +35,7 @@ static inline uint8_t gc_allocator_alloc_table_begin_pattern(enum gc_allocation_
   uint8_t trace_precisely = 0;
   uint8_t trace_none = 8;
   uint8_t trace_conservatively = 16;
+  uint8_t pinned = 16;
   switch (kind) {
   case GC_ALLOCATION_TAGGED:
     return young | trace_precisely;
@@ -43,8 +44,9 @@ static inline uint8_t gc_allocator_alloc_table_begin_pattern(enum gc_allocation_
       GC_CRASH ();
     return young | trace_conservatively;
   case GC_ALLOCATION_TAGGED_POINTERLESS:
-  case GC_ALLOCATION_UNTAGGED_POINTERLESS:
     return young | trace_none;
+  case GC_ALLOCATION_UNTAGGED_POINTERLESS:
+    return young | trace_none | pinned;
   default:
     GC_CRASH();
   }
diff --git a/libguile/whippet/src/mmc.c b/libguile/whippet/src/mmc.c
index 9b9b00046..65d076b06 100644
--- a/libguile/whippet/src/mmc.c
+++ b/libguile/whippet/src/mmc.c
@@ -617,7 +617,7 @@ grow_heap_if_necessary(struct gc_heap *heap,
   // If we cannot defragment and are making no progress but have a
   // growable heap, expand by 25% to add some headroom.
   size_t needed_headroom =
-    GC_CONSERVATIVE_TRACE
+    nofl_space_heap_has_ambiguous_edges (nofl)
     ? (progress ? 0 : nofl_active_block_count (nofl) * NOFL_BLOCK_SIZE / 4)
     : 0;
   size_t headroom = nofl_empty_block_count(nofl) * NOFL_BLOCK_SIZE;
@@ -1031,8 +1031,6 @@ compute_trace_kind(enum gc_allocation_kind kind) {
   case GC_ALLOCATION_TAGGED:
     return GC_TRACE_PRECISELY;
   case GC_ALLOCATION_UNTAGGED_CONSERVATIVE:
-    if (!GC_CONSERVATIVE_TRACE)
-      GC_CRASH ();
     return GC_TRACE_CONSERVATIVELY;
   case GC_ALLOCATION_TAGGED_POINTERLESS:
   case GC_ALLOCATION_UNTAGGED_POINTERLESS:
@@ -1359,7 +1357,7 @@ gc_init(const struct gc_options *options, struct gc_stack_addr stack_base,
     GC_ASSERT_EQ(gc_allocator_alloc_table_begin_pattern(GC_ALLOCATION_UNTAGGED_CONSERVATIVE),
                  NOFL_METADATA_BYTE_YOUNG | NOFL_METADATA_BYTE_TRACE_CONSERVATIVELY);
   GC_ASSERT_EQ(gc_allocator_alloc_table_begin_pattern(GC_ALLOCATION_UNTAGGED_POINTERLESS),
-               NOFL_METADATA_BYTE_YOUNG | NOFL_METADATA_BYTE_TRACE_NONE);
+               NOFL_METADATA_BYTE_YOUNG | NOFL_METADATA_BYTE_TRACE_NONE | NOFL_METADATA_BYTE_PINNED);
   GC_ASSERT_EQ(gc_allocator_alloc_table_end_pattern(), NOFL_METADATA_BYTE_END);
   if (GC_GENERATIONAL) {
     GC_ASSERT_EQ(gc_write_barrier_field_table_alignment(), NOFL_SLAB_SIZE);
diff --git a/libguile/whippet/src/nofl-space.h b/libguile/whippet/src/nofl-space.h
index ce62c17d3..82315aaec 100644
--- a/libguile/whippet/src/nofl-space.h
+++ b/libguile/whippet/src/nofl-space.h
@@ -984,7 +984,7 @@ static void
 nofl_clear_pinned_bits_in_block(struct nofl_block_ref block) {
   uint8_t *meta = nofl_metadata_byte_for_addr(block.addr);
   uint64_t mask = broadcast_byte (NOFL_METADATA_BYTE_PINNED);
-  for (size_t i = 0; i < NOFL_GRANULES_PER_BLOCK; i++, meta += 8) {
+  for (size_t i = 0; i < NOFL_GRANULES_PER_BLOCK; i += 8, meta += 8) {
     uint64_t vals = load_eight_aligned_bytes(meta);
     if (vals & mask)
       store_eight_aligned_bytes(meta, vals & ~mask);
@@ -1045,6 +1045,7 @@ nofl_space_set_heap_has_ambiguous_edges (struct nofl_space *space)
     // conservatively-traced.  Ideally we would have another bit here.  For now,
     // race to clear all pinned bits.
     nofl_space_clear_all_pinned_bits (space);
+    space->evacuation_minimum_reserve = space->evacuation_reserve = 0.0;
   }
 }
 
@@ -1357,15 +1358,29 @@ nofl_metadata_byte_trace_kind(struct nofl_space *space, uint8_t byte)
   switch (byte & mask) {
   case NOFL_METADATA_BYTE_TRACE_PRECISELY:
     return GC_TRACE_PRECISELY;
-  case NOFL_METADATA_BYTE_TRACE_NONE:
-    return GC_TRACE_NONE;
   case NOFL_METADATA_BYTE_TRACE_CONSERVATIVELY:
     return GC_TRACE_CONSERVATIVELY;
+  case NOFL_METADATA_BYTE_TRACE_NONE:
+    return GC_TRACE_NONE;
   default:
-    GC_CRASH();
+    /* Untagged pointerless objects are allocated with the PINNED bit,
+       because we can't relocate them, because we don't have a tag word
+       to hold the forwarding state.  Fortunately, this bit pattern is
+       different from NOFL_METADATA_BYTE_TRACE_CONSERVATIVELY; we can
+       just leave it as-is.  */
+    GC_ASSERT_EQ (byte & mask,
+                  NOFL_METADATA_BYTE_TRACE_NONE | NOFL_METADATA_BYTE_PINNED);
+    return GC_TRACE_NONE;
   }
 }
 
+static void
+nofl_assert_not_forwarded(struct gc_ref ref)
+{
+  struct gc_atomic_forward fwd = gc_atomic_forward_begin(ref);
+  GC_ASSERT_EQ(fwd.state, GC_FORWARDING_STATE_NOT_FORWARDED);
+}
+
 static void
 nofl_space_verify_sweepable_blocks(struct nofl_space *space,
                                    struct nofl_block_list *list)
@@ -1388,11 +1403,13 @@ nofl_space_verify_sweepable_blocks(struct nofl_space *space,
           granules++;
         GC_ASSERT(meta[granules - 1] & NOFL_METADATA_BYTE_END);
 
-        if (nofl_metadata_byte_trace_kind (space, byte) == GC_TRACE_PRECISELY) {
+        if (nofl_metadata_byte_trace_kind(space, byte) == GC_TRACE_PRECISELY) {
           size_t trace_bytes;
           gc_trace_object(obj, NULL, NULL, NULL, &trace_bytes);
           size_t trace_granules = nofl_size_to_granules(trace_bytes);
           GC_ASSERT_EQ(granules, trace_granules);
+
+          nofl_assert_not_forwarded(obj);
         }
 
         meta += granules;
@@ -1428,11 +1445,13 @@ nofl_space_verify_swept_blocks(struct nofl_space *space,
           granules++;
         GC_ASSERT(meta[granules - 1] & NOFL_METADATA_BYTE_END);
 
-        if (nofl_metadata_byte_trace_kind (space, byte) == GC_TRACE_PRECISELY) {
+        if (nofl_metadata_byte_trace_kind(space, byte) == GC_TRACE_PRECISELY) {
           size_t trace_bytes;
           gc_trace_object(obj, NULL, NULL, NULL, &trace_bytes);
           size_t trace_granules = nofl_size_to_granules(trace_bytes);
           GC_ASSERT_EQ(granules, trace_granules);
+
+          nofl_assert_not_forwarded(obj);
         }
 
         meta += granules;