--- a/layout/style/ServoStyleSheet.cpp
+++ b/layout/style/ServoStyleSheet.cpp
@@ -63,17 +63,21 @@ ServoStyleSheetInner::CloneFor(StyleShee
 }
 
 MOZ_DEFINE_MALLOC_SIZE_OF(ServoStyleSheetMallocSizeOf)
 
 size_t
 ServoStyleSheetInner::SizeOfIncludingThis(MallocSizeOf aMallocSizeOf) const
 {
   size_t n = aMallocSizeOf(this);
-  n += Servo_StyleSheet_SizeOfIncludingThis(ServoStyleSheetMallocSizeOf, mSheet);
+  // mSheet will be null if the parsing has not completed.
+  if (mSheet) {
+    n += Servo_StyleSheet_SizeOfIncludingThis(ServoStyleSheetMallocSizeOf,
+                                              mSheet);
+  }
   return n;
 }
 
 ServoStyleSheet::ServoStyleSheet(css::SheetParsingMode aParsingMode,
                                  CORSMode aCORSMode,
                                  net::ReferrerPolicy aReferrerPolicy,
                                  const dom::SRIMetadata& aIntegrity)
   : StyleSheet(StyleBackendType::Servo, aParsingMode)