diff --git a/src/muz/base/dl_context.cpp b/src/muz/base/dl_context.cpp
index 74c909c02..a90d420cb 100644
--- a/src/muz/base/dl_context.cpp
+++ b/src/muz/base/dl_context.cpp
@@ -188,9 +188,8 @@ namespace datalog {
         if (m_trail.get_num_scopes() == 0) {
             throw default_exception("there are no backtracking points to pop to");
         }
-        if(m_engine.get()){
-            if(get_engine() != DUALITY_ENGINE)
-            throw default_exception("operation is not supported by engine");
+        if (m_engine.get() && get_engine() != DUALITY_ENGINE) {
+            throw default_exception("pop operation is only supported by duality engine");
         }
         m_trail.pop_scope(1); 
     }
diff --git a/src/muz/fp/dl_cmds.cpp b/src/muz/fp/dl_cmds.cpp
index 874d14538..e8e93dafc 100644
--- a/src/muz/fp/dl_cmds.cpp
+++ b/src/muz/fp/dl_cmds.cpp
@@ -229,6 +229,7 @@ public:
     }
 
     virtual void prepare(cmd_context & ctx) { 
+        ctx.m(); // ensure manager is initialized.
         parametric_cmd::prepare(ctx);
         m_target   = 0; 
     }
@@ -390,6 +391,7 @@ public:
     virtual unsigned get_arity() const { return VAR_ARITY; }
 
     virtual void prepare(cmd_context & ctx) {
+        ctx.m(); // ensure manager is initialized.
         m_arg_idx = 0; 
         m_query_arg_idx = 0; 
         m_domain = 0;
@@ -450,6 +452,7 @@ public:
     virtual unsigned get_arity() const { return 2; }
 
     virtual void prepare(cmd_context & ctx) {
+        ctx.m(); // ensure manager is initialized.
         m_arg_idx = 0; 
     }
     virtual cmd_arg_kind next_arg_kind(cmd_context & ctx) const {