diff --git a/src/aero_mode.hpp b/src/aero_mode.hpp
index 2f1a1e1d..ed69cb89 100644
--- a/src/aero_mode.hpp
+++ b/src/aero_mode.hpp
@@ -148,6 +148,20 @@ struct AeroMode {
         if (mode["mode_type"] == "sampled") {
             if (mode.find("size_dist") == mode.end())
                 throw std::runtime_error("size_dist key must be set for mode_type=sampled");
+            auto sd = mode["size_dist"];
+            if (
+                sd.size() != 2 || 
+                !sd[0].is_object() || 
+                sd[0].size() != 1 || 
+                sd[1].size() != 1 ||
+                sd[0].find("num_conc") == sd[0].end() ||
+                sd[1].find("diam") == sd[1].end()
+            )
+                throw std::runtime_error("size_dist value must be an iterable of two single-element dicts (first with 'num_conc', second with 'diam' as keys)");
+            auto num_conc = *sd[0].find("num_conc");
+            auto diam = *sd[1].find("diam");
+            if (diam.size() != num_conc.size() + 1)
+                throw std::runtime_error("size_dist['num_conc'] must have len(size_dist['diam'])-1 elements");
         }
     }
 
diff --git a/tests/test_aero_dist.py b/tests/test_aero_dist.py
index adb64cac..790c9f2e 100644
--- a/tests/test_aero_dist.py
+++ b/tests/test_aero_dist.py
@@ -217,3 +217,22 @@ def test_ctor_error_on_repeated_massfrac_keys():
 
         # assert
         assert str(exc_info.value) == "mass_frac keys must be unique"
+
+    @staticmethod
+    def test_ctor_sampled_mode():
+        # arrange
+        aero_data = ppmc.AeroData(AERO_DATA_CTOR_ARG_MINIMAL)
+        ctor_arg = copy.deepcopy(AERO_DIST_CTOR_ARG_MINIMAL)
+        ctor_arg[0]["test_mode"]["mode_type"] = "sampled"
+        ctor_arg[0]["test_mode"]["size_dist"] = [
+            {"num_conc": [1, 2, 3]},
+            {"diam": [1, 2, 3, 4]},
+        ]
+
+        # act
+        sut = ppmc.AeroDist(aero_data, ctor_arg)
+
+        # assert
+        assert sut.mode(0).num_conc == sum(
+            ctor_arg[0]["test_mode"]["size_dist"][0]["num_conc"]
+        )
diff --git a/tests/test_aero_mode.py b/tests/test_aero_mode.py
index fbcc026d..250af41c 100644
--- a/tests/test_aero_mode.py
+++ b/tests/test_aero_mode.py
@@ -304,6 +304,57 @@ def test_sampled_without_size_dist():
         # assert
         assert str(exc_info.value) == "size_dist key must be set for mode_type=sampled"
 
+    @staticmethod
+    @pytest.mark.parametrize(
+        "fishy",
+        (
+            None,
+            [],
+            [{}, {}, {}],
+            [{}, []],
+            [{"diam": None}, {}],
+            [{"num_conc": None}, {}],
+            [{"diam": None, "": None}, {}],
+            [{"num_conc": None, "": None}, {}],
+        ),
+    )
+    def test_sampled_with_fishy_size_dist(fishy):
+        # arrange
+        aero_data = ppmc.AeroData(AERO_DATA_CTOR_ARG_MINIMAL)
+        fishy_ctor_arg = copy.deepcopy(AERO_MODE_CTOR_LOG_NORMAL)
+        fishy_ctor_arg["test_mode"]["mode_type"] = "sampled"
+        fishy_ctor_arg["test_mode"]["size_dist"] = fishy
+
+        # act
+        with pytest.raises(Exception) as exc_info:
+            ppmc.AeroMode(aero_data, fishy_ctor_arg)
+
+        # assert
+        assert (
+            str(exc_info.value)
+            == "size_dist value must be an iterable of two single-element dicts (first with 'num_conc', second with 'diam' as keys)"
+        )
+
+    def test_sampled_with_diam_of_different_len_than_num_conc(fishy):
+        # arrange
+        aero_data = ppmc.AeroData(AERO_DATA_CTOR_ARG_MINIMAL)
+        fishy_ctor_arg = copy.deepcopy(AERO_MODE_CTOR_LOG_NORMAL)
+        fishy_ctor_arg["test_mode"]["mode_type"] = "sampled"
+        fishy_ctor_arg["test_mode"]["size_dist"] = [
+            {"num_conc": [1, 2, 3]},
+            {"diam": [1, 2, 3]},
+        ]
+
+        # act
+        with pytest.raises(Exception) as exc_info:
+            ppmc.AeroMode(aero_data, fishy_ctor_arg)
+
+        # assert
+        assert (
+            str(exc_info.value)
+            == "size_dist['num_conc'] must have len(size_dist['diam'])-1 elements"
+        )
+
     @staticmethod
     def test_sampled():
         # arrange
@@ -319,8 +370,8 @@ def test_sampled():
                     "diam_type": "geometric",
                     "mode_type": "sampled",
                     "size_dist": [
-                        {"diam": [1, 2, 3, 4]},
                         {"num_conc": num_concs},
+                        {"diam": [1, 2, 3, 4]},
                     ],
                 }
             },