diff --git a/src/emucore/TIASurface.cxx b/src/emucore/TIASurface.cxx
index c312ffcb2..2aaca0e7b 100644
--- a/src/emucore/TIASurface.cxx
+++ b/src/emucore/TIASurface.cxx
@@ -231,7 +231,8 @@ void TIASurface::enableScanlineInterpolation(bool enable)
 void TIASurface::enablePhosphor(bool enable, int blend)
 {
   myUsePhosphor = enable;
-  myPhosphorPercent = blend / 100.0;
+  if(blend >= 0)
+    myPhosphorPercent = blend / 100.0;
   myFilter = Filter(enable ? uInt8(myFilter) | 0x01 : uInt8(myFilter) & 0x10);
 
   myTiaSurface->setDirty();