diff --git a/vm/slot_visitor.hpp b/vm/slot_visitor.hpp
index e1a2206684..92da352ea0 100644
--- a/vm/slot_visitor.hpp
+++ b/vm/slot_visitor.hpp
@@ -381,10 +381,23 @@ void slot_visitor<Fixup>::visit_context(context* ctx) {
      stacks. */
   visit_callstack(ctx);
 
-  visit_stack_elements(ctx->datastack_seg, (cell*)ctx->datastack);
-  visit_stack_elements(ctx->retainstack_seg, (cell*)ctx->retainstack);
+  cell* ds_ptr = (cell*)ctx->datastack;
+  cell* rs_ptr = (cell*)ctx->retainstack;
+  visit_stack_elements(ctx->datastack_seg, ds_ptr);
+  visit_stack_elements(ctx->retainstack_seg, rs_ptr);
   visit_object_array(ctx->context_objects,
                      ctx->context_objects + context_object_count);
+
+  /* Clear out the space not visited with a known pattern. That makes
+     it easier to see if uninitialized reads are made. */
+  #ifdef FACTOR_DEBUG
+  cell ds_clear_start = (cell)(ds_ptr + 1);
+  cell ds_clear_size = ctx->datastack_seg->end - ds_clear_start;
+  memset_cell((void*)ds_clear_start, 0xbaadbaad, ds_clear_size);
+  cell rs_clear_start = (cell)(rs_ptr + 1);
+  cell rs_clear_size = ctx->retainstack_seg->end - rs_clear_start;
+  memset_cell((void*)rs_clear_start, 0xdaabdaab, rs_clear_size);
+  #endif
 }
 
 template <typename Fixup> void slot_visitor<Fixup>::visit_contexts() {