--- a/layout/style/ServoStyleSet.cpp
+++ b/layout/style/ServoStyleSet.cpp
@@ -5,16 +5,17 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #include "mozilla/ServoStyleSet.h"
 
 #include "mozilla/ServoRestyleManager.h"
 #include "nsCSSAnonBoxes.h"
 #include "nsCSSPseudoElements.h"
 #include "nsIDocumentInlines.h"
+#include "nsPrintfCString.h"
 #include "nsStyleContext.h"
 #include "nsStyleSet.h"
 
 using namespace mozilla;
 using namespace mozilla::dom;
 
 ServoStyleSet::ServoStyleSet()
   : mPresContext(nullptr)
@@ -185,17 +186,25 @@ ServoStyleSet::ResolveAnonymousBoxStyle(
   bool skipFixup = aFlags & nsStyleSet::eSkipParentDisplayBasedStyleFixup;
 
   ServoComputedValues* parentStyle =
     aParentContext ? aParentContext->StyleSource().AsServoComputedValues()
                    : nullptr;
   RefPtr<ServoComputedValues> computedValues =
     dont_AddRef(Servo_GetComputedValuesForAnonymousBox(parentStyle, aPseudoTag,
                                                        mRawSet.get()));
-  MOZ_ASSERT(computedValues);
+#ifdef DEBUG
+  if (!computedValues) {
+    nsString pseudo;
+    aPseudoTag->ToString(pseudo);
+    NS_ERROR(nsPrintfCString("stylo: could not get anon-box: %s",
+             NS_ConvertUTF16toUTF8(pseudo).get()).get());
+    MOZ_CRASH();
+  }
+#endif
 
   return NS_NewStyleContext(aParentContext, mPresContext, aPseudoTag,
                             CSSPseudoElementType::AnonBox,
                             computedValues.forget(), skipFixup);
 }
 
 // manage the set of style sheets in the style set
 nsresult