diff --git a/riscv/mmu.cc b/riscv/mmu.cc
index b7d49d4c..1e8dd8bd 100644
--- a/riscv/mmu.cc
+++ b/riscv/mmu.cc
@@ -104,7 +104,7 @@ reg_t reg_from_bytes(size_t len, const uint8_t* bytes)
 bool mmu_t::mmio_ok(reg_t addr, access_type type)
 {
   // Disallow access to debug region when not in debug mode
-  if (addr >= DEBUG_START && addr <= DEBUG_END && !proc->state.debug_mode)
+  if (addr >= DEBUG_START && addr <= DEBUG_END && proc && !proc->state.debug_mode)
     return false;
 
   return true;