diff --git a/microcontroller/core/include/c-runtime.h b/microcontroller/core/include/c-runtime.h
index 14718bf..13cbd6a 100644
--- a/microcontroller/core/include/c-runtime.h
+++ b/microcontroller/core/include/c-runtime.h
@@ -109,14 +109,18 @@ struct gc_root_set {
     value_t values[1];
 };
 
-#define ROOT_SET(name,n)     struct { struct gc_root_set* next; uint32_t length; value_t values[n]; } name;\
-gc_init_rootset((struct gc_root_set*)&name, n);
-
-#define ROOT_SET_DECL(name,n)     struct { struct gc_root_set* next; uint32_t length; value_t values[n]; } name;
+#define ROOT_SET_DECL(name,n)     struct { struct gc_root_set* next; uint32_t length; value_t values[n]; } name
 #define ROOT_SET_INIT(name,n)     gc_init_rootset((struct gc_root_set*)&name, n);
 
 #define DELETE_ROOT_SET(name)     { gc_root_set_head = name.next; }
 
+#define ROOT_SET(name,n)    ROOT_SET_DECL(name,n); ROOT_SET_INIT(name,n)
+
+#define VALUE_UNDEF_2       VALUE_UNDEF, VALUE_UNDEF
+#define VALUE_UNDEF_3       VALUE_UNDEF, VALUE_UNDEF, VALUE_UNDEF
+#define ROOT_SET_N(name,n,initv)     ROOT_SET_DECL(name,n) \
+  = { .next = gc_root_set_head, .length = n, .values = { initv }}; gc_root_set_head = (struct gc_root_set*)&name;
+
 extern int32_t CR_SECTION try_and_catch(void (*main_function)());
 
 extern int32_t CR_SECTION safe_value_to_int(value_t v);
diff --git a/microcontroller/core/src/c-runtime.c b/microcontroller/core/src/c-runtime.c
index 28c5dd5..30aeb8f 100644
--- a/microcontroller/core/src/c-runtime.c
+++ b/microcontroller/core/src/c-runtime.c
@@ -2,10 +2,7 @@
 
 /*
   To run on a 64bit machine (for testing/debugging purpose only),
-  compile with -DTEST64.  To include test code, compile with -DTEST.
-  So,
-    cc -DTEST -DTEST64 gc.c -lm
-  will produce ./a.out that runs test code on a 64bit machine.
+  compile with -DTEST64.
 
   Typical usecase:
 
@@ -1479,6 +1476,7 @@ uint32_t gc_test_run() {
 }
 #endif
 
+// when you modify this function, also modify ROOT_SET_N macro.
 void gc_init_rootset(struct gc_root_set* set, uint32_t length) {
     set->next = gc_root_set_head;
     if (length > 0) {
diff --git a/server/src/transpiler/code-generator/c-runtime.ts b/server/src/transpiler/code-generator/c-runtime.ts
index 67a9ed8..f88a1f5 100644
--- a/server/src/transpiler/code-generator/c-runtime.ts
+++ b/server/src/transpiler/code-generator/c-runtime.ts
@@ -260,14 +260,20 @@ export const minusAnyValue = 'minus_any_value'
 export const globalRootSetName = 'global_rootset'
 
 export function makeRootSet(n: number) {
-  if (n > 0)
-    return `ROOT_SET(func_rootset, ${n})`
-  else
+  if (n < 1)
     return ''
+  else if (n == 1)
+    return 'ROOT_SET_N(func_rootset,1,VALUE_UNDEF)'
+  else if (n == 2)
+    return 'ROOT_SET_N(func_rootset,2,VALUE_UNDEF_2)'
+  else if (n == 3)
+    return 'ROOT_SET_N(func_rootset,3,VALUE_UNDEF_3)'
+  else
+    return `ROOT_SET(func_rootset,${n})`
 }
 
 export function declareRootSet(name: string, n: number) {
-  return `ROOT_SET_DECL(${name}, ${n})`
+  return `ROOT_SET_DECL(${name}, ${n});`
 }
 
 export function initRootSet(name: string, n: number) {