diff --git a/src/BlockTracer.h b/src/BlockTracer.h
index 40d80da1a..a18c8df4d 100644
--- a/src/BlockTracer.h
+++ b/src/BlockTracer.h
@@ -28,6 +28,9 @@ public:
 	class cCallbacks abstract
 	{
 	public:
+		// Force a virtual destructor in descendants:
+		virtual ~cCallbacks() {}
+		
 		/** Called on each block encountered along the path, including the first block (path start)
 		When this callback returns true, the tracing is aborted.
 		*/
diff --git a/src/HTTPServer/EnvelopeParser.h b/src/HTTPServer/EnvelopeParser.h
index 6430fbebf..866bed11d 100644
--- a/src/HTTPServer/EnvelopeParser.h
+++ b/src/HTTPServer/EnvelopeParser.h
@@ -19,6 +19,9 @@ public:
 	class cCallbacks
 	{
 	public:
+		// Force a virtual destructor in descendants:
+		virtual ~cCallbacks() {}
+		
 		/// Called when a full header line is parsed
 		virtual void OnHeaderLine(const AString & a_Key, const AString & a_Value) = 0;
 	} ;
diff --git a/src/HTTPServer/HTTPFormParser.h b/src/HTTPServer/HTTPFormParser.h
index a554ca5a4..f2a509cb8 100644
--- a/src/HTTPServer/HTTPFormParser.h
+++ b/src/HTTPServer/HTTPFormParser.h
@@ -36,6 +36,9 @@ public:
 	class cCallbacks
 	{
 	public:
+		// Force a virtual destructor in descendants:
+		virtual ~cCallbacks() {}
+		
 		/// Called when a new file part is encountered in the form data
 		virtual void OnFileStart(cHTTPFormParser & a_Parser, const AString & a_FileName) = 0;
 		
diff --git a/src/HTTPServer/MultipartParser.h b/src/HTTPServer/MultipartParser.h
index d853929ed..77695fe4a 100644
--- a/src/HTTPServer/MultipartParser.h
+++ b/src/HTTPServer/MultipartParser.h
@@ -22,6 +22,9 @@ public:
 	class cCallbacks
 	{
 	public:
+		// Force a virtual destructor in descendants:
+		virtual ~cCallbacks() {}
+		
 		/// Called when a new part starts
 		virtual void OnPartStart(void) = 0;