diff --git a/src/ast/normal_forms/nnf.cpp b/src/ast/normal_forms/nnf.cpp
index 09fccf135..3baf4e4ea 100644
--- a/src/ast/normal_forms/nnf.cpp
+++ b/src/ast/normal_forms/nnf.cpp
@@ -225,6 +225,16 @@ struct nnf::imp {
             m_cache_result(other.m_cache_result),
             m_spos(other.m_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) {            
+        }
+            
     };
 
     // There are four caches: