--- a/dom/animation/KeyframeEffectReadOnly.cpp
+++ b/dom/animation/KeyframeEffectReadOnly.cpp
@@ -518,20 +518,29 @@ KeyframeEffectReadOnly::EnsureBaseStyles
   if (!mTarget) {
     return;
   }
 
   mBaseStyleValuesForServo.Clear();
 
   nsPresContext* presContext =
     nsContentUtils::GetContextForContent(mTarget->mElement);
-  MOZ_ASSERT(presContext,
-             "nsPresContext should not be nullptr since this EnsureBaseStyles "
-             "supposed to be called right after getting computed values with "
-             "a valid nsPresContext");
+  // If |aProperties| is empty we're not going to dereference |presContext| so
+  // we don't care if it is nullptr.
+  //
+  // We could just return early when |aProperties| is empty and save looking up
+  // the pres context, but that won't save any effort normally since we don't
+  // call this function if we have no keyframes to begin with. Furthermore, the
+  // case where |presContext| is nullptr is so rare (we've only ever seen in
+  // fuzzing, and even then we've never been able to reproduce it reliably)
+  // it's not worth the runtime cost of an extra branch.
+  MOZ_ASSERT(presContext || aProperties.IsEmpty(),
+             "Typically presContext should not be nullptr but if it is"
+             " we should have also failed to calculate the computed values"
+             " passed-in as aProperties");
 
   RefPtr<ServoStyleContext> baseStyleContext;
   for (const AnimationProperty& property : aProperties) {
     EnsureBaseStyle(property,
                     presContext,
                     aComputedValues,
                     baseStyleContext);
   }