diff --git a/js/src/jit/MIRGraph.cpp b/js/src/jit/MIRGraph.cpp
index b54a98a..7739ea8 100644
--- a/js/src/jit/MIRGraph.cpp
+++ b/js/src/jit/MIRGraph.cpp
@@ -391,16 +391,17 @@ MBasicBlock::NewAsmJS(MIRGraph& graph, const CompileInfo& info, MBasicBlock* pre
 }
 
 MBasicBlock::MBasicBlock(MIRGraph& graph, const CompileInfo& info, BytecodeSite* site, Kind kind)
   : unreachable_(false),
     graph_(graph),
     info_(info),
     predecessors_(graph.alloc()),
     stackPosition_(info_.firstStackSlot()),
+    domIndex_(UINT32_MAX),
     numDominated_(0),
     pc_(site->pc()),
     lir_(nullptr),
     callerResumePoint_(nullptr),
     entryResumePoint_(nullptr),
     outerResumePoint_(nullptr),
     successorWithPhis_(nullptr),
     positionInPhiSuccessor_(0),
diff --git a/js/src/jit/MIRGraph.h b/js/src/jit/MIRGraph.h
index 7cbdd56..34af1ea 100644
--- a/js/src/jit/MIRGraph.h
+++ b/js/src/jit/MIRGraph.h
@@ -344,16 +344,17 @@ class MBasicBlock : public TempObject, public InlineListNode<MBasicBlock>
         return id_;
     }
     uint32_t numPredecessors() const {
         return predecessors_.length();
     }
 
     uint32_t domIndex() const {
         MOZ_ASSERT(!isDead());
+        MOZ_ASSERT(domIndex_ != UINT32_MAX);
         return domIndex_;
     }
     void setDomIndex(uint32_t d) {
         domIndex_ = d;
     }
 
     MBasicBlock* getPredecessor(uint32_t i) const {
         return predecessors_[i];

