diff --git a/openbr/plugins/frames.cpp b/openbr/plugins/frames.cpp
index f5f8609..fb7f507 100644
--- a/openbr/plugins/frames.cpp
+++ b/openbr/plugins/frames.cpp
@@ -27,12 +27,14 @@ private:
         (void) data;
     }
 
-    void projectUpdate(const Template &src, Template &dst)
+    void projectUpdate(const TemplateList &src, TemplateList &dst)
     {
         buffer.append(src);
         if (buffer.size() < n) return;
-        foreach (const Template &t, buffer) dst.append(t);
-        dst.file = buffer.takeFirst().file;
+        Template out;
+        foreach (const Template &t, buffer) out.append(t);
+        out.file = buffer.takeFirst().file;
+        dst.append(out);
     }
 
     void finalize(TemplateList & output)
diff --git a/openbr/plugins/opticalflow.cpp b/openbr/plugins/opticalflow.cpp
index 5246ad5..89c2044 100644
--- a/openbr/plugins/opticalflow.cpp
+++ b/openbr/plugins/opticalflow.cpp
@@ -34,7 +34,7 @@ class OpticalFlowTransform : public UntrainableMetaTransform
     void project(const Template &src, Template &dst) const
     {
         // get the two images put there by AggregateFrames
-        if (src.size() < 2) return;
+        if (src.size() != 2) qFatal("Optical Flow requires two images.");
         Mat prevImg = src[0], nextImg = src[1], flow, flowOneCh;
         if (src[0].channels() != 1) OpenCVUtils::cvtGray(src[0], prevImg);
         if (src[1].channels() != 1) OpenCVUtils::cvtGray(src[1], nextImg);