diff --git a/dom/base/nsViewportInfo.cpp b/dom/base/nsViewportInfo.cpp
index 6c86b7f0a92c..8c01dafdedac 100644
--- a/dom/base/nsViewportInfo.cpp
+++ b/dom/base/nsViewportInfo.cpp
@@ -11,6 +11,11 @@
 using namespace mozilla;
 
 void nsViewportInfo::ConstrainViewportValues() {
+  // Non-positive zoom factors can produce NaN or negative viewport sizes,
+  // so we better be sure our constraints will produce positive zoom factors.
+  MOZ_ASSERT(mMinZoom > CSSToScreenScale(0.0f), "zoom factor must be positive");
+  MOZ_ASSERT(mMaxZoom > CSSToScreenScale(0.0f), "zoom factor must be positive");
+
   if (mDefaultZoom > mMaxZoom) {
     mDefaultZoomValid = false;
     mDefaultZoom = mMaxZoom;
diff --git a/layout/base/MobileViewportManager.cpp b/layout/base/MobileViewportManager.cpp
index 1010cd3ad1e0..97c70ad677a4 100644
--- a/layout/base/MobileViewportManager.cpp
+++ b/layout/base/MobileViewportManager.cpp
@@ -219,6 +219,10 @@ void MobileViewportManager::UpdateResolution(
   CSSToLayoutDeviceScale cssToDev = mContext->CSSToDevPixelScale();
   LayoutDeviceToLayerScale res(mContext->GetResolution());
   CSSToScreenScale zoom = ResolutionToZoom(res, cssToDev);
+  // Non-positive zoom factors can produce NaN or negative viewport sizes,
+  // so we better be sure we've got a positive zoom factor.
+  MOZ_ASSERT(zoom > CSSToScreenScale(0.0f), "zoom factor must be positive");
+
   Maybe<CSSToScreenScale> newZoom;
 
   ScreenIntSize compositionSize = GetCompositionSize(aDisplaySize);
@@ -391,6 +395,10 @@ void MobileViewportManager::UpdateResolution(
 
   // If the zoom has changed, update the pres shell resolution accordingly.
   if (newZoom) {
+    // Non-positive zoom factors can produce NaN or negative viewport sizes,
+    // so we better be sure we've got a positive zoom factor.
+    MOZ_ASSERT(*newZoom > CSSToScreenScale(0.0f),
+               "zoom factor must be positive");
     LayoutDeviceToLayerScale resolution = ZoomToResolution(*newZoom, cssToDev);
     MVM_LOG("%p: setting resolution %f\n", this, resolution.scale);
     mContext->SetResolutionAndScaleTo(resolution.scale);