--- a/layout/generic/nsFloatManager.cpp
+++ b/layout/generic/nsFloatManager.cpp
@@ -698,31 +698,34 @@ nsFloatManager::FloatInfo::FloatInfo(nsI
     mShapeInfo = MakeUnique<BoxShapeInfo>(shapeBoxRect, mFrame);
   } else if (shapeOutside.GetType() == StyleShapeSourceType::Shape) {
     StyleBasicShape* const basicShape = shapeOutside.GetBasicShape();
 
     switch (basicShape->GetShapeType()) {
       case StyleBasicShapeType::Polygon:
         // Bug 1326409 - Implement the rendering of basic shape polygon()
         // for CSS shape-outside.
-        break;
+        return;
       case StyleBasicShapeType::Circle:
       case StyleBasicShapeType::Ellipse:
         mShapeInfo =
           ShapeInfo::CreateCircleOrEllipse(basicShape, rect, aWM, aContainerSize);
         break;
       case StyleBasicShapeType::Inset:
         // Bug 1326407 - Implement the rendering of basic shape inset() for
         // CSS shape-outside.
-        break;
+        return;
     }
   } else {
     MOZ_ASSERT_UNREACHABLE("Unknown StyleShapeSourceType!");
   }
 
+  MOZ_ASSERT(mShapeInfo,
+             "All shape-outside values except none should have mShapeInfo!");
+
   // Translate the shape to the same origin as nsFloatManager.
   mShapeInfo->Translate(aLineLeft, aBlockStart);
 }
 
 #ifdef NS_BUILD_REFCNT_LOGGING
 nsFloatManager::FloatInfo::FloatInfo(FloatInfo&& aOther)
   : mFrame(Move(aOther.mFrame))
   , mLeftBEnd(Move(aOther.mLeftBEnd))