diff --git a/src/aero_dist.hpp b/src/aero_dist.hpp
index cab50f17..8bd752c9 100644
--- a/src/aero_dist.hpp
+++ b/src/aero_dist.hpp
@@ -51,6 +51,8 @@ struct AeroDist {
     {
         if (!InputGimmick::unique_keys(json))
             throw std::runtime_error("Mode names must be unique");
+        for (const auto &mode : json)
+            AeroMode::check_mode_json(mode.begin().value());
         
         GimmickGuard<InputGimmick> guard(json, "", "mode_name", 1);
         f_aero_dist_from_json(ptr.f_arg_non_const(), aero_data->ptr.f_arg_non_const());
diff --git a/src/aero_mode.hpp b/src/aero_mode.hpp
index e8ac50c8..145bb044 100644
--- a/src/aero_mode.hpp
+++ b/src/aero_mode.hpp
@@ -128,13 +128,25 @@ struct AeroMode {
     AeroMode(AeroData &aero_data, const nlohmann::json &json) :
         ptr(f_aero_mode_ctor, f_aero_mode_dtor)
     {
-        if (json.size() != 1)
-            throw std::runtime_error("Single element expected");
-
+        if (json.size() != 1 || !json.is_object() || !json.begin().value().is_object())
+            throw std::runtime_error("Single-element dict expected with mode name as key and mode params dict as value");
+        check_mode_json(json.begin().value());
         GimmickGuard<InputGimmick> guard(json, "", "mode_name");
         f_aero_mode_from_json(ptr.f_arg_non_const(), aero_data.ptr.f_arg_non_const());
     }
 
+    static void check_mode_json(const nlohmann::json &mode) {
+        for (auto key : std::set<std::string>({"mass_frac", "mode_type"})) // TODO #320: more...
+            if (mode.find(key) == mode.end())
+                throw std::runtime_error("mode parameters dict must include key '" + key + "'");
+        auto mass_frac = mode["mass_frac"];
+            
+        if (!mass_frac.is_array()) // TODO #320: check if all are single-element dicts
+            throw std::runtime_error("mass_frac value must be a list of single-element dicts");
+        if (!InputGimmick::unique_keys(mass_frac))
+            throw std::runtime_error("mass_frac keys must be unique");
+    }
+
     static auto get_num_conc(const AeroMode &self){
        double val;
        f_aero_mode_get_num_conc(self.ptr.f_arg(), &val);
diff --git a/tests/test_aero_dist.py b/tests/test_aero_dist.py
index b02dc0f9..d8f03a01 100644
--- a/tests/test_aero_dist.py
+++ b/tests/test_aero_dist.py
@@ -163,3 +163,19 @@ def test_ctor_multimode_error_on_repeated_mode_names():
 
         # assert
         assert str(exc_info.value) == "Mode names must be unique"
+
+    @staticmethod
+    def test_ctor_error_on_repeated_massfrac_keys():
+        # arrange
+        aero_data = ppmc.AeroData(AERO_DATA_CTOR_ARG_MINIMAL)
+        fishy_ctor_arg = copy.deepcopy(AERO_DIST_CTOR_ARG_MINIMAL)
+        fishy_ctor_arg[0]["test_mode"]["mass_frac"].append(
+            fishy_ctor_arg[0]["test_mode"]["mass_frac"][0]
+        )
+
+        # act
+        with pytest.raises(Exception) as exc_info:
+            ppmc.AeroDist(aero_data, fishy_ctor_arg)
+
+        # assert
+        assert str(exc_info.value) == "mass_frac keys must be unique"
diff --git a/tests/test_aero_mode.py b/tests/test_aero_mode.py
index e927e674..523e20fa 100644
--- a/tests/test_aero_mode.py
+++ b/tests/test_aero_mode.py
@@ -4,6 +4,8 @@
 # Authors: https://github.com/open-atmos/PyPartMC/graphs/contributors                              #
 ####################################################################################################
 
+import copy
+
 import numpy as np
 import pytest
 
@@ -242,7 +244,7 @@ def test_set_name():
     def test_ctor_fails_with_multiple_modes():
         # arrange
         aero_data = ppmc.AeroData(AERO_DATA_CTOR_ARG_MINIMAL)
-        fishy_ctor_arg = AERO_MODE_CTOR_LOG_NORMAL
+        fishy_ctor_arg = copy.deepcopy(AERO_MODE_CTOR_LOG_NORMAL)
         fishy_ctor_arg["xxx"] = fishy_ctor_arg["test_mode"]
 
         # act
@@ -250,17 +252,18 @@ def test_ctor_fails_with_multiple_modes():
             ppmc.AeroMode(aero_data, fishy_ctor_arg)
 
         # assert
-        assert str(exc_info.value) == "Single element expected"
+        assert (
+            str(exc_info.value)
+            == "Single-element dict expected with mode name as key and mode params dict as value"
+        )
 
     @staticmethod
     def test_ctor_fails_with_nonunique_mass_fracs():
-        pytest.skip("TODO #240")
-
         # arrange
         aero_data = ppmc.AeroData(AERO_DATA_CTOR_ARG_MINIMAL)
-        fishy_ctor_arg = AERO_MODE_CTOR_LOG_NORMAL
+        fishy_ctor_arg = copy.deepcopy(AERO_MODE_CTOR_LOG_NORMAL)
         fishy_ctor_arg["test_mode"]["mass_frac"].append(
-            fishy_ctor_arg["test_mode"]["mass_frac"]
+            AERO_MODE_CTOR_LOG_NORMAL["test_mode"]["mass_frac"][0]
         )
 
         # act
@@ -268,4 +271,16 @@ def test_ctor_fails_with_nonunique_mass_fracs():
             ppmc.AeroMode(aero_data, fishy_ctor_arg)
 
         # assert
-        assert str(exc_info.value) == ""
+        assert str(exc_info.value) == "mass_frac keys must be unique"
+
+    @staticmethod
+    def test_segfault_case():  # TODO #319
+        pytest.skip()
+
+        aero_data = ppmc.AeroData(AERO_DATA_CTOR_ARG_MINIMAL)
+        fishy_ctor_arg = copy.deepcopy(AERO_MODE_CTOR_LOG_NORMAL)
+        fishy_ctor_arg["test_mode"]["mass_frac"].append(
+            fishy_ctor_arg["test_mode"]["mass_frac"]
+        )
+        print(fishy_ctor_arg)
+        ppmc.AeroMode(aero_data, fishy_ctor_arg)