diff --git a/runtime.c b/runtime.c
index b0ccc85..28ed2cc 100644
--- a/runtime.c
+++ b/runtime.c
@@ -2576,6 +2576,9 @@ C_regparm C_word C_fcall C_mutate(C_word *slot, C_word val)
 
       mutation_stack_limit = mutation_stack_bottom + newmssize;
       mutation_stack_top = mutation_stack_bottom + mssize;
+
+      /* Heap needs to resize at least this many bytes plus its normal growth */
+      heap_size += bytes;
     }
 
     *(mutation_stack_top++) = slot;