diff --git a/tket/src/Utils/Expression.cpp b/tket/src/Utils/Expression.cpp
index e24263b098..d81ca4617c 100644
--- a/tket/src/Utils/Expression.cpp
+++ b/tket/src/Utils/Expression.cpp
@@ -14,8 +14,11 @@
 
 #include "Expression.hpp"
 
+#include <optional>
+
 #include "Constants.hpp"
 #include "Symbols.hpp"
+#include "symengine/symengine_exception.h"
 
 namespace tket {
 
@@ -57,7 +60,11 @@ std::optional<double> eval_expr(const Expr& e) {
   if (!SymEngine::free_symbols(e).empty()) {
     return std::nullopt;
   } else {
-    return SymEngine::eval_double(e);
+    try {
+      return SymEngine::eval_double(e);
+    } catch (SymEngine::NotImplementedError&) {
+      return std::nullopt;
+    }
   }
 }
 
diff --git a/tket/tests/Circuit/test_Boxes.cpp b/tket/tests/Circuit/test_Boxes.cpp
index 54903436c8..9d75f9efeb 100644
--- a/tket/tests/Circuit/test_Boxes.cpp
+++ b/tket/tests/Circuit/test_Boxes.cpp
@@ -380,6 +380,12 @@ SCENARIO("Pauli gadgets", "[boxes]") {
     Eigen::MatrixXcd u = tket_sim::get_unitary(c);
     REQUIRE((u - Eigen::Matrix4cd::Identity()).cwiseAbs().sum() < ERR_EPS);
   }
+  GIVEN("complex coefficient") {
+    Expr ei{SymEngine::I};
+    PauliExpBox pebox({Pauli::Z}, ei);
+    Expr p = pebox.get_phase();
+    REQUIRE(p == ei);
+  }
 }
 
 SCENARIO("box daggers", "[boxes]") {