diff --git a/vm/heap.cpp b/vm/heap.cpp
index 9cd59c9139..1689af3ee4 100644
--- a/vm/heap.cpp
+++ b/vm/heap.cpp
@@ -13,8 +13,9 @@ void heap::clear_free_list()
 
 heap::heap(bool secure_gc_, cell size) : secure_gc(secure_gc_)
 {
+	if(size > (1L << (sizeof(cell) * 8 - 6))) fatal_error("Heap too large",size);
 	seg = new segment(align_page(size));
-	if(!seg) fatal_error("Out of memory in new_heap",size);
+	if(!seg) fatal_error("Out of memory in heap allocator",size);
 	clear_free_list();
 }
 
diff --git a/vm/layouts.hpp b/vm/layouts.hpp
index 25db79b00a..988fc99ec5 100644
--- a/vm/layouts.hpp
+++ b/vm/layouts.hpp
@@ -64,9 +64,9 @@ inline static cell align8(cell a)
 
 #define TYPE_COUNT 15
 
-/* Not a real type, but code_block's type field can be set to this */
-#define PIC_TYPE 42
-#define FREE_BLOCK_TYPE 69
+/* Not real types, but code_block's type can be set to this */
+#define PIC_TYPE 16
+#define FREE_BLOCK_TYPE 17
 
 /* Constants used when floating-point trap exceptions are thrown */
 enum
@@ -213,16 +213,16 @@ struct heap_block
 			header &= ~1;
 	}
 
-	cell type() { return (header >> 1) & 0x7f; }
+	cell type() { return (header >> 1) & 0x1f; }
 	void set_type(cell type)
 	{
-		header = ((header & ~(0x7f << 1)) | (type << 1));
+		header = ((header & ~(0x1f << 1)) | (type << 1));
 	}
 
-	cell size() { return (header >> 8); }
+	cell size() { return (header >> 6); }
 	void set_size(cell size)
 	{
-		header = (header & 0xff) | (size << 8);
+		header = (header & 0x2f) | (size << 6);
 	}
 };