diff --git a/src/emucore/tia/frame-manager/FrameLayoutDetector.cxx b/src/emucore/tia/frame-manager/FrameLayoutDetector.cxx
index 5ae3be83b..6cbfb7c7d 100644
--- a/src/emucore/tia/frame-manager/FrameLayoutDetector.cxx
+++ b/src/emucore/tia/frame-manager/FrameLayoutDetector.cxx
@@ -244,8 +244,9 @@ void FrameLayoutDetector::finalizeFrame()
   // 1.0 (>=312) and added to PAL and (inverted) NTSC sums.
   constexpr double ODD_PENALTY = 0.5; // guessed value :)
   const double palFrame = BSPF::clamp(((myCurrentFrameFinalLines % 2) ? ODD_PENALTY : 1.0)
-    * static_cast<double>(myCurrentFrameFinalLines - frameLinesNTSC)
+    * (static_cast<double>(myCurrentFrameFinalLines) - static_cast<double>(frameLinesNTSC))
     / static_cast<double>(frameLinesPAL - frameLinesNTSC), 0.0, 1.0);
+
   myPalFrameSum += palFrame;
   myNtscFrameSum += 1.0 - palFrame;
   //cerr << myCurrentFrameFinalLines << ", " << palFrame << ", " << myPalFrameSum << ", " << myNtscFrameSum << '\n';