diff --git a/vm.c b/vm.c
index 764512cfec..9db8b67826 100644
--- a/vm.c
+++ b/vm.c
@@ -163,17 +163,22 @@ switchkvm(void)
 void
 switchuvm(struct proc *p)
 {
+  if(p == 0)
+    panic("switchuvm: no process");
+  if(p->kstack == 0)
+    panic("switchuvm: no kstack");
+  if(p->pgdir == 0)
+    panic("switchuvm: no pgdir");
+
   pushcli();
   cpu->gdt[SEG_TSS] = SEG16(STS_T32A, &cpu->ts, sizeof(cpu->ts)-1, 0);
   cpu->gdt[SEG_TSS].s = 0;
   cpu->ts.ss0 = SEG_KDATA << 3;
-  cpu->ts.esp0 = (uint)proc->kstack + KSTACKSIZE;
+  cpu->ts.esp0 = (uint)p->kstack + KSTACKSIZE;
   // setting IOPL=0 in eflags *and* iomb beyond the tss segment limit
   // forbids I/O instructions (e.g., inb and outb) from user space
   cpu->ts.iomb = (ushort) 0xFFFF;
   ltr(SEG_TSS << 3);
-  if(p->pgdir == 0)
-    panic("switchuvm: no pgdir");
   lcr3(V2P(p->pgdir));  // switch to process's address space
   popcli();
 }