diff --git a/src/Weasel.Postgresql.Tests/PostgresqlProviderTests.cs b/src/Weasel.Postgresql.Tests/PostgresqlProviderTests.cs
index 84a3d3b..51b1a71 100644
--- a/src/Weasel.Postgresql.Tests/PostgresqlProviderTests.cs
+++ b/src/Weasel.Postgresql.Tests/PostgresqlProviderTests.cs
@@ -85,6 +85,34 @@ public void ipnetwork_resolves_to_cidr()
             .ShouldBe("cidr");
     }
 
+    [Fact]
+    public void ipnetwork_is_claimed_by_exactly_one_mapping()
+    {
+        // Guards the test above. IPNetwork used to be declared on both the cidr and the inet
+        // mapping, and GetTypeMapping breaks a tie with LastOrDefault over a Cache backed by
+        // an ImHashMap -- so "cidr" was winning on hash layout, not on anything declared.
+        // weasel#405.
+        NpgsqlTypeMapper.Mappings
+            .Count(mapping => mapping.ClrTypes.Contains(typeof(IPNetwork)))
+            .ShouldBe(1);
+    }
+
+    [Fact]
+    public void no_clr_type_is_claimed_by_more_than_one_mapping()
+    {
+        // Any CLR type reachable from two mappings has an order-dependent, effectively
+        // arbitrary resolution. Keep that structurally impossible rather than relying on
+        // enumeration order. weasel#405.
+        var doubleClaimed = NpgsqlTypeMapper.Mappings
+            .SelectMany(mapping => mapping.ClrTypes.Select(clrType => (clrType, mapping)))
+            .GroupBy(x => x.clrType)
+            .Where(g => g.Count() > 1)
+            .Select(g => $"{g.Key.Name} <- {string.Join(", ", g.Select(x => x.mapping.NpgsqlDbType))}")
+            .ToArray();
+
+        doubleClaimed.ShouldBeEmpty();
+    }
+
     [Fact]
     public void canonicizesql_supports_tabs_as_whitespace()
     {
diff --git a/src/Weasel.Postgresql/NpgsqlTypeMapping.cs b/src/Weasel.Postgresql/NpgsqlTypeMapping.cs
index 3919c48..ae394c7 100644
--- a/src/Weasel.Postgresql/NpgsqlTypeMapping.cs
+++ b/src/Weasel.Postgresql/NpgsqlTypeMapping.cs
@@ -82,9 +82,12 @@ public class NpgsqlTypeMapper
         {NpgsqlDbType.Multirange | NpgsqlDbType.TimestampTz, new NpgsqlTypeMapping(NpgsqlDbType.Multirange | NpgsqlDbType.TimestampTz, DbType.Object, "tstzmultirange")},
 
         // Network types
+        // IPNetwork belongs to cidr only. It used to be listed on inet as well, and which of
+        // the two won was decided by LastOrDefault over a hash-ordered ImHashMap -- i.e. by
+        // accident rather than by declaration. See weasel#405.
         {NpgsqlDbType.Cidr, new NpgsqlTypeMapping(NpgsqlDbType.Cidr, DbType.Object, "cidr", typeof(IPNetwork))},
         {NpgsqlDbType.Inet, new NpgsqlTypeMapping(NpgsqlDbType.Inet, DbType.Object, "inet", typeof(IPAddress),
-            typeof((IPAddress Address, int Subnet)), typeof(NpgsqlInet), typeof(IPNetwork), IPAddress.Loopback.GetType())},
+            typeof((IPAddress Address, int Subnet)), typeof(NpgsqlInet), IPAddress.Loopback.GetType())},
         {NpgsqlDbType.MacAddr, new NpgsqlTypeMapping(NpgsqlDbType.MacAddr, DbType.Object, "macaddr", typeof(PhysicalAddress))},
         {NpgsqlDbType.MacAddr8, new NpgsqlTypeMapping(NpgsqlDbType.MacAddr8, DbType.Object, "macaddr8")},