diff --git a/eval.c b/eval.c
index c641894b..f4efba99 100644
--- a/eval.c
+++ b/eval.c
@@ -392,7 +392,7 @@ sexp sexp_make_child_context (sexp ctx, sexp lambda) {
                                     sexp_context_stack(ctx),
                                     sexp_context_env(ctx),
                                     0,
-                                    sexp_context_heap(ctx)->max_size);
+                                    sexp_context_max_size(ctx));
   if (! sexp_exceptionp(res)) {
     sexp_context_lambda(res) = lambda;
     sexp_context_top(res) = sexp_context_top(ctx);
diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 0da16d32..bd7b029a 100644
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -813,9 +813,11 @@ SEXP_API sexp sexp_make_unsigned_integer(sexp ctx, sexp_luint_t x);
 #if ! SEXP_USE_BOEHM
 SEXP_API sexp_heap sexp_global_heap;
 #endif
-#define sexp_context_heap(ctx)  sexp_global_heap
+#define sexp_context_heap(ctx)     sexp_global_heap
+#define sexp_context_max_size(ctx) 0
 #else
-#define sexp_context_heap(ctx)  ((ctx)->value.context.heap)
+#define sexp_context_heap(ctx)     ((ctx)->value.context.heap)
+#define sexp_context_max_size(ctx) sexp_context_heap(ctx)->max_size
 #endif
 
 #if SEXP_USE_GLOBAL_SYMBOLS