diff --git a/dom/animation/KeyframeEffectReadOnly.cpp b/dom/animation/KeyframeEffectReadOnly.cpp
index ed8d6e9b0e0f..ce5e0378a7e1 100644
--- a/dom/animation/KeyframeEffectReadOnly.cpp
+++ b/dom/animation/KeyframeEffectReadOnly.cpp
@@ -277,6 +277,8 @@ KeyframeEffectReadOnly::UpdateProperties(nsStyleContext* aStyleContext)
   // Skip updating properties when we are composing style.
   // FIXME: Bug 1324966. Drop this check once we have a function to get
   // nsStyleContext without resolving animating style.
+  MOZ_DIAGNOSTIC_ASSERT(!mIsComposingStyle,
+                        "Should not be called while processing ComposeStyle()");
   if (mIsComposingStyle) {
     return;
   }
@@ -465,6 +467,8 @@ KeyframeEffectReadOnly::ComposeStyle(
   RefPtr<AnimValuesStyleRule>& aStyleRule,
   const nsCSSPropertyIDSet& aPropertiesToSkip)
 {
+  MOZ_DIAGNOSTIC_ASSERT(!mIsComposingStyle,
+                        "Should not be called recursively");
   if (mIsComposingStyle) {
     return;
   }