diff --git a/core/mem.c b/core/mem.c
index f442aa7..823cc6c 100644
--- a/core/mem.c
+++ b/core/mem.c
@@ -136,6 +136,7 @@ int unrefPhyPage(paddr_t addr)
     if (!mem) {
         return -1;
     }
+    assert(mem->ref >0);
     mem->ref--;
     if (mem->ref == 0) {
         allocatedPage--;
diff --git a/core/mem.h b/core/mem.h
index c876879..6e68948 100644
--- a/core/mem.h
+++ b/core/mem.h
@@ -11,7 +11,7 @@ extern uint32_t __ld_kernel_end;
 
 struct memDesc {
     paddr_t phy_addr;
-    long ref;
+    unsigned long ref;
     struct memDesc *next, *prev;
 };