--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -1274,17 +1274,25 @@ ServoStyleSet::EnsureUniqueInnerOnCSSShe
   // Bug 1290276 will replicate the nsStyleSet work of checking
   // a nsBindingManager
 
   while (!queue.IsEmpty()) {
     uint32_t idx = queue.Length() - 1;
     StyleSheet* sheet = queue[idx];
     queue.RemoveElementAt(idx);
 
-    sheet->EnsureUniqueInner();
+    // Only call EnsureUniqueInner for complete sheets. If we do call it on
+    // incomplete sheets, we'll cause problems when the sheet is actually
+    // loaded. We don't care about incomplete sheets here anyway, because this
+    // method is only invoked by nsPresContext::EnsureSafeToHandOutCSSRules.
+    // The CSSRule objects we are handing out won't contain any rules derived
+    // from incomplete sheets (because they aren't yet applied in styling).
+    if (sheet->IsComplete()) {
+      sheet->EnsureUniqueInner();
+    }
 
     // Enqueue all the sheet's children.
     sheet->AppendAllChildSheets(queue);
   }
 
   if (mNeedsRestyleAfterEnsureUniqueInner) {
     // TODO(emilio): We could make this faster if needed tracking the specific
     // origins and all that, but the only caller of this doesn't seem to really

--- a/layout/style/nsStyleSet.cpp
+++ b/layout/style/nsStyleSet.cpp
@@ -2724,17 +2724,25 @@ nsStyleSet::EnsureUniqueInnerOnCSSSheets
     }
   }
 
   while (!queue.IsEmpty()) {
     uint32_t idx = queue.Length() - 1;
     StyleSheet* sheet = queue[idx];
     queue.RemoveElementAt(idx);
 
-    sheet->EnsureUniqueInner();
+    // Only call EnsureUniqueInner for complete sheets. If we do call it on
+    // incomplete sheets, we'll cause problems when the sheet is actually
+    // loaded. We don't care about incomplete sheets here anyway, because this
+    // method is only invoked by nsPresContext::EnsureSafeToHandOutCSSRules.
+    // The CSSRule objects we are handing out won't contain any rules derived
+    // from incomplete sheets (because they aren't yet applied in styling).
+    if (sheet->IsComplete()) {
+      sheet->EnsureUniqueInner();
+    }
 
     // Enqueue all the sheet's children.
     sheet->AppendAllChildSheets(queue);
   }
 
   bool res = mNeedsRestyleAfterEnsureUniqueInner;
   mNeedsRestyleAfterEnsureUniqueInner = false;
   return res;