diff --git a/changelogs/client_server/newsfragments/3636.clarification b/changelogs/client_server/newsfragments/3636.clarification
new file mode 100644
index 00000000..a1efa517
--- /dev/null
+++ b/changelogs/client_server/newsfragments/3636.clarification
@@ -0,0 +1 @@
+Clarify how to interpret missing one-time key counts.
diff --git a/content/client-server-api/modules/end_to_end_encryption.md b/content/client-server-api/modules/end_to_end_encryption.md
index 990a7751..aeded979 100644
--- a/content/client-server-api/modules/end_to_end_encryption.md
+++ b/content/client-server-api/modules/end_to_end_encryption.md
@@ -1645,7 +1645,7 @@ a way to identify the server's support for fallback keys.
 | Parameter                        | Type               | Description                                                                                                            |
 |----------------------------------|--------------------|------------------------------------------------------------------------------------------------------------------------|
 | device_lists                     | DeviceLists        | Optional. Information on e2e device updates. Note: only present on an incremental sync.                                |
-| device_one_time_keys_count       | {string: integer}  | Optional. For each key algorithm, the number of unclaimed one-time keys currently held on the server for this device.  |
+| device_one_time_keys_count       | {string: integer}  | Optional. For each key algorithm, the number of unclaimed one-time keys currently held on the server for this device.  If an algorithm is unlisted, the count for that algorithm is assumed to be zero.  If this entire parameter is missing, the count for all algorithms is assumed to be zero.  |
 | device_unused_fallback_key_types | [string]           | **Required.** The unused fallback key algorithms.                                                                      |
 
 `DeviceLists`
diff --git a/data/api/client-server/keys.yaml b/data/api/client-server/keys.yaml
index 357df233..116a661d 100644
--- a/data/api/client-server/keys.yaml
+++ b/data/api/client-server/keys.yaml
@@ -148,6 +148,8 @@ paths:
                 description: |-
                   For each key algorithm, the number of unclaimed one-time keys
                   of that type currently held on the server for this device.
+                  If an algorithm is not listed, the count for that algorithm
+                  is to be assumed zero.
                 example:
                   curve25519: 10
                   signed_curve25519: 20