diff --git a/common/ImageBuffer.cpp b/common/ImageBuffer.cpp
index ce1ef37b..e3ced97f 100644
--- a/common/ImageBuffer.cpp
+++ b/common/ImageBuffer.cpp
@@ -109,12 +109,13 @@ ZeroCopyReturnValue *ImageBuffer::GetImageSlot() {
         auto i = free_slots.front();
         free_slots.pop();
 
+        if (v[i].status == ImageBufferEntryStatus::Idle)
+            --slots_idle;
         v[i].status = ImageBufferEntryStatus::InPreparation;
         v[i].image_number = INT64_MIN;
         v[i].image_size = 0;
         v[i].indexed = false;
 
-        --slots_idle;
         ++slots_preparation;
 
         // Wait for any preview thread currently reading this one