diff --git a/project/include/Surface.h b/project/include/Surface.h
index ea5035cab..1eac69187 100644
--- a/project/include/Surface.h
+++ b/project/include/Surface.h
@@ -40,7 +40,9 @@ extern bool RpiHardwareDecodeJPeg(I420Callback inCallback, void *inUserData, con
 
 struct IAppDataCallback
 {
+   virtual void beginCallbacks() = 0;
    virtual void onAppData(int marker, const uint8 *inBytes,int inLen ) = 0;
+   virtual void endCallbacks() = 0;
 };
 
 
diff --git a/project/src/common/ExternalInterface.cpp b/project/src/common/ExternalInterface.cpp
index 2e7ac4929..1095513d5 100644
--- a/project/src/common/ExternalInterface.cpp
+++ b/project/src/common/ExternalInterface.cpp
@@ -4136,8 +4136,9 @@ TEXT_PROP_GET_IDX_PRIME(line_offset,LineOffset,int);
 struct ValueAppDataCallback : public IAppDataCallback
 {
    value cbValue;
+   bool  unblocked;
 
-   ValueAppDataCallback(value inValue) : cbValue(inValue) { }
+   ValueAppDataCallback(value inValue) : cbValue(inValue), unblocked(false) { }
 
    IAppDataCallback *get()
    {
@@ -4146,13 +4147,31 @@ struct ValueAppDataCallback : public IAppDataCallback
       return this;
    }
 
+   void beginCallbacks()
+   {
+      unblocked =  gc_try_unblocking();
+   }
+
    void onAppData(int marker, const uint8 *inBytes,int inLen )
    {
       ByteArray byteArray(inLen);
       uint8 *bytes = byteArray.Bytes();
       if (bytes)
          memcpy(bytes, inBytes, inLen);
-      val_call2( cbValue, alloc_int(marker), byteArray.mValue );
+      try
+      {
+         val_call2( cbValue, alloc_int(marker), byteArray.mValue );
+      }
+      catch(...)
+      {
+         // Ignore?
+      }
+   }
+
+   void endCallbacks()
+   {
+      if (unblocked)
+          gc_enter_blocking();
    }
 
 };
diff --git a/project/src/common/SurfaceIO.cpp b/project/src/common/SurfaceIO.cpp
index f93e72bad..5f048c397 100644
--- a/project/src/common/SurfaceIO.cpp
+++ b/project/src/common/SurfaceIO.cpp
@@ -270,10 +270,15 @@ static Surface *TryJPEG(FILE *inFile,const uint8 *inData, int inDataLen, IAppDat
    if (onAppData)
    {
       jpeg_saved_marker_ptr marker = cinfo.marker_list;
-      while(marker)
+      if (marker)
       {
-         onAppData->onAppData(marker->marker, marker->data, marker->data_length);
-         marker = marker->next;
+         onAppData->beginCallbacks();
+         while(marker)
+         {
+            onAppData->onAppData(marker->marker, marker->data, marker->data_length);
+            marker = marker->next;
+         }
+         onAppData->endCallbacks();
       }
    }