diff --git a/moto/ec2/models.py b/moto/ec2/models.py
index f6f53683a..05224a45d 100755
--- a/moto/ec2/models.py
+++ b/moto/ec2/models.py
@@ -2919,12 +2919,14 @@ class SpotFleetBackend(object):
         return spot_requests
 
     def modify_spot_fleet_request(self, spot_fleet_request_id, target_capacity, terminate_instances):
+        if target_capacity < 0:
+            raise ValueError('Cannot reduce spot fleet capacity below 0')
         spot_fleet_request = self.spot_fleet_requests[spot_fleet_request_id]
-        delta = target_capacity - spot_fleet_request.target_capacity
+        delta = target_capacity - spot_fleet_request.fulfilled_capacity
         spot_fleet_request.target_capacity = target_capacity
         if delta > 0:
             spot_fleet_request.create_spot_requests(delta)
-        elif delta < 0 and terminate_instances == 'default':
+        elif delta < 0 and terminate_instances == 'Default':
             spot_fleet_request.terminate_instances()
         return True
 
diff --git a/moto/ec2/responses/spot_fleets.py b/moto/ec2/responses/spot_fleets.py
index 167618215..81d1e0146 100644
--- a/moto/ec2/responses/spot_fleets.py
+++ b/moto/ec2/responses/spot_fleets.py
@@ -32,7 +32,7 @@ class SpotFleets(BaseResponse):
     def modify_spot_fleet_request(self):
         spot_fleet_request_id = self._get_param("SpotFleetRequestId")
         target_capacity = self._get_int_param("TargetCapacity")
-        terminate_instances = self._get_param("ExcessCapacityTerminationPolicy", if_none="default")
+        terminate_instances = self._get_param("ExcessCapacityTerminationPolicy", if_none="Default")
         successful = self.ec2_backend.modify_spot_fleet_request(
             spot_fleet_request_id, target_capacity, terminate_instances)
         template = self.response_template(MODIFY_SPOT_FLEET_REQUEST_TEMPLATE)
@@ -65,10 +65,10 @@ REQUEST_SPOT_FLEET_TEMPLATE = """<RequestSpotFleetResponse xmlns="http://ec2.ama
     <spotFleetRequestId>{{ request.id }}</spotFleetRequestId>
 </RequestSpotFleetResponse>"""
 
-MODIFY_SPOT_FLEET_REQUEST_TEMPLATE = """<RequestSpotFleetResponse xmlns="http://ec2.amazonaws.com/doc/2016-09-15/">
+MODIFY_SPOT_FLEET_REQUEST_TEMPLATE = """<ModifySpotFleetRequestResponse xmlns="http://ec2.amazonaws.com/doc/2016-09-15/">
     <requestId>21681fea-9987-aef3-2121-example</requestId>
-    <return>{{ successful }}</return>
-</RequestSpotFleetResponse>"""
+    <return>{{ 'true' if successful else 'false' }}</return>
+</ModifySpotFleetRequestResponse>"""
 
 DESCRIBE_SPOT_FLEET_TEMPLATE = """<DescribeSpotFleetRequestsResponse xmlns="http://ec2.amazonaws.com/doc/2016-09-15/">
     <requestId>4d68a6cc-8f2e-4be1-b425-example</requestId>
diff --git a/tests/test_ec2/test_spot_fleet.py b/tests/test_ec2/test_spot_fleet.py
index a8737a17c..442f77a94 100644
--- a/tests/test_ec2/test_spot_fleet.py
+++ b/tests/test_ec2/test_spot_fleet.py
@@ -239,6 +239,32 @@ def test_modify_spot_fleet_request_down_no_terminate():
     spot_fleet_config['FulfilledCapacity'].should.equal(6)
 
 
+@mock_ec2
+def test_modify_spot_fleet_request_down_odd():
+    conn = boto3.client("ec2", region_name='us-west-2')
+    subnet_id = get_subnet_id(conn)
+
+    spot_fleet_res = conn.request_spot_fleet(
+        SpotFleetRequestConfig=spot_config(subnet_id),
+    )
+    spot_fleet_id = spot_fleet_res['SpotFleetRequestId']
+
+    conn.modify_spot_fleet_request(
+        SpotFleetRequestId=spot_fleet_id, TargetCapacity=7)
+    conn.modify_spot_fleet_request(
+        SpotFleetRequestId=spot_fleet_id, TargetCapacity=5)
+
+    instance_res = conn.describe_spot_fleet_instances(
+        SpotFleetRequestId=spot_fleet_id)
+    instances = instance_res['ActiveInstances']
+    len(instances).should.equal(3)
+
+    spot_fleet_config = conn.describe_spot_fleet_requests(
+        SpotFleetRequestIds=[spot_fleet_id])['SpotFleetRequestConfigs'][0]['SpotFleetRequestConfig']
+    spot_fleet_config['TargetCapacity'].should.equal(5)
+    spot_fleet_config['FulfilledCapacity'].should.equal(6)
+
+
 @mock_ec2
 def test_modify_spot_fleet_request_down():
     conn = boto3.client("ec2", region_name='us-west-2')