--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -864,16 +864,17 @@ CreateObserversForAnimatedGlyphs(gfxText
  */
 class BuildTextRunsScanner {
 public:
   BuildTextRunsScanner(nsPresContext* aPresContext, DrawTarget* aDrawTarget,
       nsIFrame* aLineContainer, nsTextFrame::TextRunType aWhichTextRun) :
     mCurrentFramesAllSameTextRun(nullptr),
     mDrawTarget(aDrawTarget),
     mLineContainer(aLineContainer),
+    mCommonAncestorWithLastFrame(nullptr),
     mMissingFonts(aPresContext->MissingFontRecorder()),
     mBidiEnabled(aPresContext->BidiEnabled()),
     mSkipIncompleteTextRuns(false),
     mWhichTextRun(aWhichTextRun),
     mNextRunContextInfo(nsTextFrameUtils::INCOMING_NONE),
     mCurrentRunContextInfo(nsTextFrameUtils::INCOMING_NONE) {
     ResetRunInfo();
   }