diff --git a/layout/generic/nsPageFrame.cpp b/layout/generic/nsPageFrame.cpp
index 7963a735d22..3cbd7e1057e 100644
--- a/layout/generic/nsPageFrame.cpp
+++ b/layout/generic/nsPageFrame.cpp
@@ -110,7 +110,9 @@ NS_IMETHODIMP nsPageFrame::Reflow(nsPresContext*          aPresContext,
                     avHeight);
     float scale = aPresContext->GetPageScale();
     maxSize.width = NSToCoordCeil(maxSize.width / scale);
-    maxSize.height = NSToCoordCeil(maxSize.height / scale);
+    if (maxSize.height != NS_UNCONSTRAINEDSIZE) {
+      maxSize.height = NSToCoordCeil(maxSize.height / scale);
+    }
     // Get the number of Twips per pixel from the PresContext
     nscoord onePixelInTwips = nsPresContext::CSSPixelsToAppUnits(1);
     // insurance against infinite reflow, when reflowing less than a pixel