diff --git a/src/newgrf_config.h b/src/newgrf_config.h
--- a/src/newgrf_config.h
+++ b/src/newgrf_config.h
@@ -113,6 +113,9 @@ struct GRFError {
 	GRFError(StringID severity, StringID message = 0);
 	GRFError(const GRFError &error);
 
+	/* Remove the copy assignment, as the default implementation will not do the right thing. */
+	GRFError &operator=(GRFError &rhs) = delete;
+
 	std::string custom_message; ///< Custom message (if present)
 	std::string data;           ///< Additional data for message and custom_message
 	StringID message;           ///< Default message
@@ -154,6 +157,9 @@ struct GRFConfig : ZeroedMemoryAllocator
 	GRFConfig(const GRFConfig &config);
 	~GRFConfig();
 
+	/* Remove the copy assignment, as the default implementation will not do the right thing. */
+	GRFConfig &operator=(GRFConfig &rhs) = delete;
+
 	GRFIdentifier ident;                        ///< grfid and md5sum to uniquely identify newgrfs
 	uint8 original_md5sum[16];                  ///< MD5 checksum of original file if only a 'compatible' file was loaded
 	char *filename;                             ///< Filename - either with or without full path