--- a/layout/generic/nsPageFrame.cpp
+++ b/layout/generic/nsPageFrame.cpp
@@ -606,17 +606,18 @@ nsPageBreakFrame::Reflow(nsPresContext* 
                          nsReflowStatus&          aStatus)
 {
   DO_GLOBAL_REFLOW_COUNT("nsPageBreakFrame");
   DISPLAY_REFLOW(aPresContext, this, aReflowState, aDesiredSize, aStatus);
 
   // Override reflow, since we don't want to deal with what our
   // computed values are.
   aDesiredSize.width = GetIntrinsicWidth();
-  aDesiredSize.height = aReflowState.availableHeight;
+  aDesiredSize.height = (aReflowState.availableHeight == NS_UNCONSTRAINEDSIZE ?
+                         0 : aReflowState.availableHeight);
   // round the height down to the nearest pixel
   aDesiredSize.height -=
     aDesiredSize.height % nsPresContext::CSSPixelsToAppUnits(1);
 
   // Note: not using NS_FRAME_FIRST_REFLOW here, since it's not clear whether
   // DidReflow will always get called before the next Reflow() call.
   mHaveReflowed = PR_TRUE;
   aStatus = NS_FRAME_COMPLETE;