diff --git a/Makefile b/Makefile
index 103c3307..ed0e1093 100644
--- a/Makefile
+++ b/Makefile
@@ -41,7 +41,7 @@ debug: $(GST_TARGET)
 	gdb $(GST_TARGET)
 
 valgrind: $(GST_TARGET)
-	valgrind ./$(GST_TARGET)
+	valgrind --leak-check=full ./$(GST_TARGET)
 
 clean:
 	rm $(GST_TARGET) || true
diff --git a/core/compile.c b/core/compile.c
index 71418bbb..72e2b2a9 100644
--- a/core/compile.c
+++ b/core/compile.c
@@ -1140,7 +1140,7 @@ GstFunction *gst_compiler_compile(GstCompiler *c, GstValue form) {
 /* Stl */
 /***/
 
-/* GC mark mark all memory used by the compiler */
+/* GC mark all memory used by the compiler */
 static void gst_compiler_mark(Gst *vm, void *data, uint32_t len) {
     SlotTracker *st;
     GstScope *scope;
@@ -1149,16 +1149,19 @@ static void gst_compiler_mark(Gst *vm, void *data, uint32_t len) {
     	return;
     /* Mark compiler */
     gst_mark_value(vm, gst_wrap_buffer(c->buffer));
-    /* Mark trackers */
+    /* Mark trackers - the trackers themselves are all on the stack. */
     st = (SlotTracker *) c->trackers;
     while (st) {
-		gst_mark_mem(vm, st->slots);
+        if (st->slots)
+            gst_mark_mem(vm, st->slots);
 		st = st->next;
     }
     /* Mark scopes */
     scope = c->tail;
     while (scope) {
-        gst_mark_mem(vm, scope->freeHeap);
+        gst_mark_mem(vm, scope);
+        if (scope->freeHeap)
+            gst_mark_mem(vm, scope->freeHeap);
         gst_mark_value(vm, gst_wrap_array(scope->literalsArray));
         gst_mark_value(vm, gst_wrap_table(scope->locals));
         gst_mark_value(vm, gst_wrap_table(scope->literals));
@@ -1173,7 +1176,7 @@ static const GstUserType gst_stl_compilertype = {
 	"std.compiler",
 	NULL,
 	NULL,
-	NULL,
+    NULL,
 	&gst_compiler_mark
 };
 
diff --git a/core/gc.c b/core/gc.c
index eb1c705b..ca328f51 100644
--- a/core/gc.c
+++ b/core/gc.c
@@ -176,8 +176,8 @@ void gst_mark(Gst *vm, GstValueUnion x, GstType type) {
             break;
 
         case GST_USERDATA:
-            if (gc_header(x.string - sizeof(GstUserdataHeader))->color != vm->black) {
-                GstUserdataHeader *h = (GstUserdataHeader *)x.pointer - 1;
+            if (gc_header(gst_udata_header(x.pointer))->color != vm->black) {
+                GstUserdataHeader *h = gst_udata_header(x.pointer);
                 gc_header(h)->color = vm->black;
                 if (h->type->gcmark)
                     h->type->gcmark(vm, x.pointer, h->size);
diff --git a/core/util.c b/core/util.c
index 1c5be32e..2f2bf546 100644
--- a/core/util.c
+++ b/core/util.c
@@ -79,7 +79,7 @@ void *gst_check_userdata(Gst *vm, uint32_t i, const GstUserType *type) {
     GstValue x = gst_arg(vm, i);
     GstUserdataHeader *h;
     if (x.type != GST_USERDATA) return NULL;
-    h = ((GstUserdataHeader *)x.data.pointer) - 1;
+    h = gst_udata_header(x.data.pointer);
     if (h->type != type) return NULL;
     return x.data.pointer;
 }
@@ -111,7 +111,6 @@ GstValue gst_cmodule_struct(Gst *vm, const GstModuleItem *mod) {
         ++m;
     }
     return gst_wrap_struct(gst_struct_end(vm, st));
-    
 }
 
 void gst_module_put(Gst *vm, const char *packagename, GstValue mod) {