diff --git a/pytket/conanfile.py b/pytket/conanfile.py
index 1847139704..9e69328491 100644
--- a/pytket/conanfile.py
+++ b/pytket/conanfile.py
@@ -32,7 +32,7 @@ def package(self):
         cmake.install()
 
     def requirements(self):
-        self.requires("tket/1.2.95@tket/stable")
+        self.requires("tket/1.2.96@tket/stable")
         self.requires("tklog/0.3.3@tket/stable")
         self.requires("tkrng/0.3.3@tket/stable")
         self.requires("tkassert/0.3.4@tket/stable")
diff --git a/pytket/tests/circuit_test.py b/pytket/tests/circuit_test.py
index 0999dfafe6..7692de9c38 100644
--- a/pytket/tests/circuit_test.py
+++ b/pytket/tests/circuit_test.py
@@ -1439,6 +1439,38 @@ def test_add_circbox_with_mixed_registers() -> None:
         )
 
 
+def test_deserialization_from_junk() -> None:
+    # https://github.com/CQCL/tket/issues/1243
+    with pytest.raises(RuntimeError):
+        Circuit.from_dict(
+            {
+                "phase": "1.9999999999999998",
+                "qubits": [("q", (20057, 24021, 112, 9628, 79))],
+                "bits": [("c", (128, 3, 384))],
+                "implicit_permutation": [
+                    (("c", (1174437931,)), ("q", (0,))),
+                    (("c", (25199232,)), ("c", (29697, 126852352))),
+                ],
+                "commands": [
+                    {
+                        "op": {
+                            "type": "CCX",
+                            "conditional": {
+                                "op": {"type": "CCX"},
+                                "width": 3,
+                                "value": 2,
+                            },
+                        },
+                        "args": [],
+                    }
+                ],
+                "name": "ú\x19",
+                "created_qubits": [("c", (0,))],
+                "discarded_qubits": [("c", (0,))],
+            }
+        )
+
+
 if __name__ == "__main__":
     test_circuit_gen()
     test_symbolic_ops()
diff --git a/tket/conanfile.py b/tket/conanfile.py
index a78e09d56d..a9bfdc2851 100644
--- a/tket/conanfile.py
+++ b/tket/conanfile.py
@@ -23,7 +23,7 @@
 
 class TketConan(ConanFile):
     name = "tket"
-    version = "1.2.95"
+    version = "1.2.96"
     package_type = "library"
     license = "Apache 2"
     homepage = "https://github.com/CQCL/tket"
diff --git a/tket/src/Circuit/CommandJson.cpp b/tket/src/Circuit/CommandJson.cpp
index 55ec10ed79..9dcdc95f88 100644
--- a/tket/src/Circuit/CommandJson.cpp
+++ b/tket/src/Circuit/CommandJson.cpp
@@ -31,7 +31,7 @@ void to_json(nlohmann::json& j, const Command& com) {
   const unit_vector_t& args = com.get_args();
 
   if (sig.size() != args.size()) {
-    JsonError("Number of args does not match signature of op.");
+    throw JsonError("Number of args does not match signature of op.");
   }
 
   nlohmann::json j_args;
@@ -72,7 +72,7 @@ void from_json(const nlohmann::json& j, Command& com) {
 
   const nlohmann::json& j_args = j.at("args");
   if (sig.size() != j_args.size()) {
-    JsonError("Number of args does not match signature of op.");
+    throw JsonError("Number of args does not match signature of op.");
   }
   unit_vector_t args;
   for (std::size_t i = 0; i < sig.size(); i++) {