diff --git a/src/zm_image.h b/src/zm_image.h
index 566e2477b2..92975de4c5 100644
--- a/src/zm_image.h
+++ b/src/zm_image.h
@@ -207,7 +207,7 @@ class Image {
     const size_t buffer_size,
     const int p_buffertype);
 
-  int PopulateFrame(AVFrame *frame);
+  int PopulateFrame(AVFrame *frame) const;
 
   inline void CopyBuffer(const Image &image) {
     Assign(image);