diff --git a/include/ConflictSet.h b/include/ConflictSet.h
index a100e01..0b91d6a 100644
--- a/include/ConflictSet.h
+++ b/include/ConflictSet.h
@@ -132,6 +132,13 @@ struct __attribute__((__visibility__("default"))) ConflictSet {
 
 private:
   Impl *impl;
+#if defined(__cplusplus) && __cplusplus <= 199711L
+  /* Declared private and left undefined to prevent copying in C++98/C++03.
+     The compiler would otherwise implicitly generate public copy operations,
+     which share the opaque Impl* and cause a double-free. */
+  ConflictSet(const ConflictSet &);
+  ConflictSet &operator=(const ConflictSet &);
+#endif
 };
 } /* namespace weaselab */