diff --git a/engine/src/flutter/fml/raster_thread_merger.cc b/engine/src/flutter/fml/raster_thread_merger.cc
index 6110e683bfb..88b33dc1e77 100644
--- a/engine/src/flutter/fml/raster_thread_merger.cc
+++ b/engine/src/flutter/fml/raster_thread_merger.cc
@@ -91,7 +91,9 @@ bool RasterThreadMerger::IsOnPlatformThread() const {
   return MessageLoop::GetCurrentTaskQueueId() == platform_queue_id_;
 }
 
-bool RasterThreadMerger::IsOnRasterizingThread() const {
+bool RasterThreadMerger::IsOnRasterizingThread() {
+  std::scoped_lock lock(mutex_);
+
   if (IsMergedUnSafe()) {
     return IsOnPlatformThread();
   } else {
diff --git a/engine/src/flutter/fml/raster_thread_merger.h b/engine/src/flutter/fml/raster_thread_merger.h
index d3babbfe5d1..3ad4a4e27da 100644
--- a/engine/src/flutter/fml/raster_thread_merger.h
+++ b/engine/src/flutter/fml/raster_thread_merger.h
@@ -89,7 +89,7 @@ class RasterThreadMerger
   // Returns true if the current thread owns rasterizing.
   // When the threads are merged, platform thread owns rasterizing.
   // When un-merged, raster thread owns rasterizing.
-  bool IsOnRasterizingThread() const;
+  bool IsOnRasterizingThread();
 
   // Returns true if the current thread is the platform thread.
   bool IsOnPlatformThread() const;