diff --git a/Core/Debugger/MemBlockInfo.cpp b/Core/Debugger/MemBlockInfo.cpp
index f727d3a484..0f6be382bf 100644
--- a/Core/Debugger/MemBlockInfo.cpp
+++ b/Core/Debugger/MemBlockInfo.cpp
@@ -288,7 +288,12 @@ void MemSlabMap::Clear() {
 
 MemSlabMap::Slab *MemSlabMap::FindSlab(uint32_t addr) {
 	// Jump ahead using our index.
-	Slab *slab = heads_[addr / SLICE_SIZE];
+	size_t slabIndex = addr / SLICE_SIZE;
+	if (slabIndex >= heads_.size()) {
+		// Shouldn't happen, but apparently can.
+		return nullptr;
+	}
+	Slab *slab = heads_[slabIndex];
 	// We often move forward, so check the last find.
 	if (lastFind_->start > slab->start && lastFind_->start <= addr)
 		slab = lastFind_;