--- a/gfx/layers/apz/testutil/APZTestData.h
+++ b/gfx/layers/apz/testutil/APZTestData.h
@@ -81,17 +81,16 @@ private:
   DataStore mPaints;
   DataStore mRepaintRequests;
 
   void LogTestDataImpl(DataStore& aDataStore,
                        SequenceNumber aSequenceNumber,
                        ViewID aScrollId,
                        const std::string& aKey,
                        const std::string& aValue) {
-    MOZ_ASSERT(gfxPrefs::APZTestLoggingEnabled(), "don't call me");
     auto bucketIterator = aDataStore.find(aSequenceNumber);
     if (bucketIterator == aDataStore.end()) {
       MOZ_ASSERT(false, "LogTestDataImpl called with nonexistent sequence number");
       return;
     }
     Bucket& bucket = bucketIterator->second;
     ScrollFrameData& scrollFrameData = bucket[aScrollId];  // create if doesn't exist
     MOZ_ASSERT(scrollFrameData.find(aKey) == scrollFrameData.end()
@@ -100,18 +99,19 @@ private:
   }
 };
 
 // A helper class for logging data for a paint.
 class APZPaintLogHelper {
 public:
   APZPaintLogHelper(APZTestData* aTestData, SequenceNumber aPaintSequenceNumber)
     : mTestData(aTestData),
-      mPaintSequenceNumber(aPaintSequenceNumber)
-  {}
+      mPaintSequenceNumber(aPaintSequenceNumber) {
+    MOZ_ASSERT(gfxPrefs::APZTestLoggingEnabled(), "don't call me");
+  }
 
   template <typename Value>
   void LogTestData(FrameMetrics::ViewID aScrollId,
                    const std::string& aKey,
                    const Value& aValue) const {
     if (mTestData) {  // avoid stringifying if mTestData == nullptr
       LogTestData(aScrollId, aKey, ToString(aValue));
     }