--- a/dom/canvas/CanvasRenderingContext2D.cpp
+++ b/dom/canvas/CanvasRenderingContext2D.cpp
@@ -2695,33 +2695,33 @@ GetFontStyleForServo(Element* aElement, 
 
   ServoStyleSet* styleSet = aPresShell->StyleSet();
 
   RefPtr<ComputedStyle> parentStyle;
   // have to get a parent ComputedStyle for inherit-like relative
   // values (2em, bolder, etc.)
   if (aElement && aElement->IsInComposedDoc()) {
     parentStyle = nsComputedDOMStyle::GetComputedStyle(aElement, nullptr);
-    if (!parentStyle) {
-      // The flush killed the shell, so we couldn't get any meaningful style
-      // back.
-      aError.Throw(NS_ERROR_FAILURE);
-      return nullptr;
-    }
   } else {
     RefPtr<RawServoDeclarationBlock> declarations =
       CreateFontDeclarationForServo(NS_LITERAL_STRING("10px sans-serif"),
                                     aPresShell->GetDocument());
     MOZ_ASSERT(declarations);
-
-    parentStyle = aPresShell->StyleSet()->
-      ResolveForDeclarations(nullptr, declarations);
-  }
-
-  MOZ_RELEASE_ASSERT(parentStyle, "Should have a valid parent style");
+    if (!aPresShell->IsDestroying()) {
+      parentStyle =
+        aPresShell->StyleSet()->ResolveForDeclarations(nullptr, declarations);
+    }
+  }
+
+  if (!parentStyle) {
+    // Either the flush killed the shell, so we couldn't get any meaningful
+    // style back, or the shell was already destroying.
+    aError.Throw(NS_ERROR_FAILURE);
+    return nullptr;
+  }
 
   MOZ_ASSERT(!aPresShell->IsDestroying(),
              "We should have returned an error above if the presshell is "
              "being destroyed.");
 
   RefPtr<ComputedStyle> sc =
     styleSet->ResolveForDeclarations(parentStyle, declarations);