diff --git a/src/ast/normal_forms/nnf.cpp b/src/ast/normal_forms/nnf.cpp
index f625095d0..6363d374a 100644
--- a/src/ast/normal_forms/nnf.cpp
+++ b/src/ast/normal_forms/nnf.cpp
@@ -216,6 +216,15 @@ struct nnf::imp {
             m_cache_result(cache_res),
             m_spos(spos) {
         }
+        frame(frame & other):
+            m_curr(other.m_curr),
+            m_i(other.m_i),
+            m_pol(other.m_pol),
+            m_in_q(other.m_in_q),
+            m_new_child(other.m_new_child),
+            m_cache_result(other.m_cache_result),
+            m_spos(other.m_spos) {            
+        }
         //frame():m_curr(*(ast_manager*)(nullptr)) {
         //}
     };