diff --git a/src/Frame.h b/src/Frame.h
index 721c7b3f96..19a86590bf 100644
--- a/src/Frame.h
+++ b/src/Frame.h
@@ -62,7 +62,9 @@ class Frame {
     return *this;
   }
 
-  explicit operator bool() const { return m_data; }
+  explicit operator bool() const {
+    return m_data && m_data->pixelFormat != VideoMode::kUnknown;
+  }
 
   operator llvm::StringRef() const {
     if (!m_data) return llvm::StringRef{};
diff --git a/src/SourceImpl.h b/src/SourceImpl.h
index 166f5beb66..5fff01d434 100644
--- a/src/SourceImpl.h
+++ b/src/SourceImpl.h
@@ -118,6 +118,9 @@ class SourceImpl {
  protected:
   void PutFrame(VideoMode::PixelFormat pixelFormat, llvm::StringRef data,
                 Frame::Time time);
+  void PutError(llvm::StringRef msg, Frame::Time time) {
+    PutFrame(VideoMode::kUnknown, msg, time);
+  }
 
   // Notification functions for corresponding atomics
   virtual void NumSinksChanged() = 0;