diff --git a/src/cmd_context/extra_cmds/dbg_cmds.cpp b/src/cmd_context/extra_cmds/dbg_cmds.cpp
index 5a628ce58..c6f0b479a 100644
--- a/src/cmd_context/extra_cmds/dbg_cmds.cpp
+++ b/src/cmd_context/extra_cmds/dbg_cmds.cpp
@@ -377,8 +377,8 @@ public:
 };
 
 class get_interpolant_cmd : public cmd {
-    expr* m_a;
-    expr* m_b;
+    scoped_ptr<expr_ref> m_a;
+    scoped_ptr<expr_ref> m_b;
 public:
     get_interpolant_cmd():cmd("get-interpolant") {}
     char const * get_usage() const override { return "<expr> <expr>"; }
@@ -388,17 +388,24 @@ public:
         return CPK_EXPR; 
     }
     void set_next_arg(cmd_context& ctx, expr * arg) override { 
-        if (m_a == nullptr) 
-            m_a = arg; 
+        ast_manager& m = ctx.m();
+        if (!m.is_bool(arg))
+            throw default_exception("argument to interpolation is not Boolean");
+        if (!m_a) 
+            m_a = alloc(expr_ref, arg, m); 
         else 
-            m_b = arg; 
+            m_b = alloc(expr_ref, arg, m); 
     }
     void prepare(cmd_context & ctx) override { m_a = nullptr; m_b = nullptr;  }
     void execute(cmd_context & ctx) override { 
         ast_manager& m = ctx.m();
         qe::interpolator mbi(m);
+        if (!m_a || !m_b)
+            throw default_exception("interpolation requires two arguments");
+        if (!m.is_bool(*m_a) || !m.is_bool(*m_b))
+            throw default_exception("interpolation requires two Boolean arguments");
         expr_ref itp(m);
-         mbi.pogo(ctx.get_solver_factory(), m_a, m_b, itp);
+        mbi.pogo(ctx.get_solver_factory(), *m_a, *m_b, itp);
         ctx.regular_stream() << itp << "\n";
     }
 };
diff --git a/src/qe/qe_mbi.cpp b/src/qe/qe_mbi.cpp
index 0261ad979..b261e44be 100644
--- a/src/qe/qe_mbi.cpp
+++ b/src/qe/qe_mbi.cpp
@@ -536,6 +536,7 @@ namespace qe {
         th_rewriter rewrite(m);
         rewrite(a);
         rewrite(b);
+        TRACE("interpolator", tout << a << " " << b << "\n");        
         solver_ref sA = sf(m, p, false /* no proofs */, true, true, symbol::null);
         solver_ref sB = sf(m, p, false /* no proofs */, true, true, symbol::null);
         solver_ref sNotA = sf(m, p, false /* no proofs */, true, true, symbol::null);
diff --git a/src/solver/assertions/asserted_formulas.cpp b/src/solver/assertions/asserted_formulas.cpp
index c6a8093ff..e1345fe92 100644
--- a/src/solver/assertions/asserted_formulas.cpp
+++ b/src/solver/assertions/asserted_formulas.cpp
@@ -154,6 +154,7 @@ void asserted_formulas::assert_expr(expr * e, proof * _in_pr) {
     force_push();
     proof_ref  in_pr(_in_pr, m), pr(_in_pr, m);
     expr_ref   r(e, m);
+    SASSERT(m.is_bool(e));
 
     if (inconsistent())
         return;