diff --git a/data/mods/TEST_DATA/monstergroups.json b/data/mods/TEST_DATA/monstergroups.json
index 67aa5eb28867c..2939296502a4f 100644
--- a/data/mods/TEST_DATA/monstergroups.json
+++ b/data/mods/TEST_DATA/monstergroups.json
@@ -19,5 +19,42 @@
       { "monster": "mon_test_shearable", "weight": 50, "event": "christmas" },
       { "monster": "mon_test_bovine", "weight": 50, "event": "christmas" }
     ]
+  },
+  {
+    "name": "test_default_monster",
+    "type": "monstergroup",
+    "default": "mon_null",
+    "monsters": [
+      { "monster": "mon_test_non_shearable", "weight": 30 },
+      { "monster": "mon_test_non_shearable", "weight": 10, "pack_size": [ 2, 4 ] },
+      { "monster": "mon_test_shearable", "weight": 5 },
+      { "monster": "mon_test_shearable", "weight": 3, "pack_size": [ 1, 3 ] },
+      { "monster": "mon_test_bovine", "weight": 5 },
+      { "monster": "mon_test_bovine", "weight": 3, "pack_size": [ 1, 3 ] },
+      { "monster": "mon_test_CBM", "weight": 5 },
+      { "monster": "mon_test_CBM", "weight": 3, "pack_size": [ 1, 3 ] },
+      { "monster": "mon_test_speed_desc_base", "weight": 10 },
+      { "monster": "mon_test_speed_desc_base_immobile", "weight": 10 }
+    ]
+  },
+  {
+    "//": "Extends the test_default_monster group",
+    "name": "test_default_monster",
+    "type": "monstergroup",
+    "monsters": [
+      { "monster": "mon_test_CBM", "weight": 50 },
+      { "group": "test_event_mongroup", "weight": 10 },
+      { "group": "test_event_only", "weight": 5 }
+    ]
+  },
+  {
+    "name": "test_group_default_monster",
+    "type": "monstergroup",
+    "default": "mon_null",
+    "monsters": [
+      { "monster": "mon_test_CBM", "weight": 50 },
+      { "group": "test_event_mongroup", "weight": 10 },
+      { "group": "test_event_only", "weight": 5 }
+    ]
   }
 ]
diff --git a/src/mongroup.cpp b/src/mongroup.cpp
index a90b112358584..83236408b0f12 100644
--- a/src/mongroup.cpp
+++ b/src/mongroup.cpp
@@ -417,8 +417,14 @@ void MonsterGroupManager::LoadMonsterGroup( const JsonObject &jo )
         g = monsterGroupMap[g.name];
         extending = true;
     }
+    bool explicit_def_null = false;
     if( !extending || jo.has_string( "default" ) ) {
         g.defaultMonster = mtype_id( jo.get_string( "default", "mon_null" ) );
+        if( jo.has_string( "default" ) && g.defaultMonster == mon_null ) {
+            explicit_def_null = true;
+        }
+    } else if( extending && !jo.has_string( "default" ) && g.defaultMonster == mon_null ) {
+        explicit_def_null = true;
     }
     g.is_animal = jo.get_bool( "is_animal", false );
     if( jo.has_array( "monsters" ) ) {
@@ -490,7 +496,7 @@ void MonsterGroupManager::LoadMonsterGroup( const JsonObject &jo )
             g.monsters.push_back( new_mon_group );
         }
         // If no default monster specified, use the highest frequency spawn as the default
-        if( g.defaultMonster == mon_null ) {
+        if( g.defaultMonster == mon_null && !explicit_def_null ) {
             g.defaultMonster = max_freq.first;
         }
     }
diff --git a/src/overmap.cpp b/src/overmap.cpp
index 2578c80c5a6df..4442b4116b3f4 100644
--- a/src/overmap.cpp
+++ b/src/overmap.cpp
@@ -6065,8 +6065,9 @@ void overmap::place_mongroups()
                     }
                 }
                 if( swamp_count >= 25 ) {
+                    float norm_factor = std::abs( GROUP_SWAMP->freq_total / 1000.0f );
                     spawn_mon_group( mongroup( GROUP_SWAMP, tripoint( x * 2, y * 2, 0 ), 3,
-                                               rng( swamp_count * 8, swamp_count * 25 ) ) );
+                                               std::round( norm_factor * rng( swamp_count * 8, swamp_count * 25 ) ) ) );
                 }
             }
         }
@@ -6084,8 +6085,9 @@ void overmap::place_mongroups()
                 }
             }
             if( river_count >= 25 ) {
+                float norm_factor = std::abs( GROUP_RIVER->freq_total / 1000.0f );
                 spawn_mon_group( mongroup( GROUP_RIVER, tripoint( x * 2, y * 2, 0 ), 3,
-                                           rng( river_count * 8, river_count * 25 ) ) );
+                                           std::round( norm_factor * rng( river_count * 8, river_count * 25 ) ) ) );
             }
         }
     }
@@ -6093,9 +6095,10 @@ void overmap::place_mongroups()
     // Place the "put me anywhere" groups
     int numgroups = rng( 0, 3 );
     for( int i = 0; i < numgroups; i++ ) {
+        float norm_factor = std::abs( GROUP_WORM->freq_total / 1000.0f );
         spawn_mon_group( mongroup( GROUP_WORM, tripoint( rng( 0, OMAPX * 2 - 1 ), rng( 0,
                                    OMAPY * 2 - 1 ), 0 ),
-                                   rng( 20, 40 ), rng( 30, 50 ) ) );
+                                   rng( 20, 40 ), std::round( norm_factor * rng( 30, 50 ) ) ) );
     }
 }
 
diff --git a/tests/mongroup_test.cpp b/tests/mongroup_test.cpp
index a6447465e4e24..85fbdd65b81c4 100644
--- a/tests/mongroup_test.cpp
+++ b/tests/mongroup_test.cpp
@@ -7,6 +7,7 @@
 #include "options.h"
 #include "options_helpers.h"
 
+static const mtype_id mon_null( "mon_null" );
 static const mtype_id mon_test_CBM( "mon_test_CBM" );
 static const mtype_id mon_test_bovine( "mon_test_bovine" );
 static const mtype_id mon_test_non_shearable( "mon_test_non_shearable" );
@@ -137,4 +138,16 @@ TEST_CASE( "Event-based monsters from an event-only mongroup", "[monster][mongro
             }
         }
     }
+}
+
+TEST_CASE( "Using mon_null as mongroup default monster", "[mongroup]" )
+{
+    mongroup_id test_group1( "test_default_monster" );
+    mongroup_id test_group2( "test_group_default_monster" );
+    mongroup_id test_group3( "test_event_only" );
+    mongroup_id test_group4( "test_event_mongroup" );
+    CHECK( test_group1->defaultMonster == mon_null );
+    CHECK( test_group2->defaultMonster == mon_null );
+    CHECK( test_group3->defaultMonster == mon_null );
+    CHECK( test_group4->defaultMonster != mon_null );
 }
\ No newline at end of file