diff --git a/synapse/app/_base.py b/synapse/app/_base.py
index e1ff8f9b7c..cf4730730d 100644
--- a/synapse/app/_base.py
+++ b/synapse/app/_base.py
@@ -12,10 +12,16 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+
 import gc
 import logging
+import sys
+
+try:
+    import affinity
+except:
+    affinity = None
 
-import affinity
 from daemonize import Daemonize
 from synapse.util import PreserveLoggingContext
 from synapse.util.rlimit import change_resource_limit
@@ -78,6 +84,13 @@ def start_reactor(
         with PreserveLoggingContext():
             logger.info("Running")
             if cpu_affinity is not None:
+                if not affinity:
+                    quit_with_error(
+                        "Missing package 'affinity' required for cpu_affinity\n"
+                        "option\n\n"
+                        "Install by running:\n\n"
+                        "   pip install affinity\n\n"
+                    )
                 logger.info("Setting CPU affinity to %s" % cpu_affinity)
                 affinity.set_process_affinity_mask(0, cpu_affinity)
             change_resource_limit(soft_file_limit)
diff --git a/synapse/python_dependencies.py b/synapse/python_dependencies.py
index 630e92c90e..7052333c19 100644
--- a/synapse/python_dependencies.py
+++ b/synapse/python_dependencies.py
@@ -40,7 +40,6 @@ REQUIREMENTS = {
     "pymacaroons-pynacl": ["pymacaroons"],
     "msgpack-python>=0.3.0": ["msgpack"],
     "phonenumbers>=8.2.0": ["phonenumbers"],
-    "affinity": ["affinity"],
 }
 CONDITIONAL_REQUIREMENTS = {
     "web_client": {
@@ -59,6 +58,9 @@ CONDITIONAL_REQUIREMENTS = {
     "psutil": {
         "psutil>=2.0.0": ["psutil>=2.0.0"],
     },
+    "affinity": {
+        "affinity": ["affinity"],
+    },
 }