diff --git a/paths/population_frequentUser_count.yaml b/paths/population_frequentUser_count.yaml
index b99fd5f..77b1ede 100644
--- a/paths/population_frequentUser_count.yaml
+++ b/paths/population_frequentUser_count.yaml
@@ -61,6 +61,8 @@ get:
                     - {visits: 1, frequency: 355} 
                     - {visits: 2, frequency: 240}
                     - {visits: 4, frequency: 25}
+    400:
+      $ref: '../zoo-api.yaml#/components/responses/ReqFailure'
     401:
       $ref: '../zoo-api.yaml#/components/responses/JwtFailure'
     403:
diff --git a/paths/population_frequentUser_patient.yaml b/paths/population_frequentUser_patient.yaml
index 05f638a..1b494f8 100644
--- a/paths/population_frequentUser_patient.yaml
+++ b/paths/population_frequentUser_patient.yaml
@@ -79,6 +79,8 @@ get:
                         - S74.02
                         - Y07.00
                         - K87.00
+    400:
+      $ref: '../zoo-api.yaml#/components/responses/ReqFailure'
     401:
       $ref: '../zoo-api.yaml#/components/responses/JwtFailure'
     403:
diff --git a/paths/population_frequentUser_patients.yaml b/paths/population_frequentUser_patients.yaml
index c75310f..91a3c85 100644
--- a/paths/population_frequentUser_patients.yaml
+++ b/paths/population_frequentUser_patients.yaml
@@ -98,6 +98,8 @@ get:
                         n:
                           type: number
                           example: 54
+    400:
+      $ref: '../zoo-api.yaml#/components/responses/ReqFailure'
     401:
       $ref: '../zoo-api.yaml#/components/responses/JwtFailure'
     403:
diff --git a/paths/population_statistics.yaml b/paths/population_statistics.yaml
index 2f58733..872b4e4 100644
--- a/paths/population_statistics.yaml
+++ b/paths/population_statistics.yaml
@@ -86,7 +86,7 @@ get:
                       description: "The number of unique patients in the group."
                       example: 7
     400:
-      description: "Common HTTP semantics, see the response body."
+      $ref: '../zoo-api.yaml#/components/responses/ReqFailure'
     401:
       $ref: '../zoo-api.yaml#/components/responses/JwtFailure'
     403:
diff --git a/paths/population_statistics_deprived.yaml b/paths/population_statistics_deprived.yaml
index 9feee68..0a3bb72 100644
--- a/paths/population_statistics_deprived.yaml
+++ b/paths/population_statistics_deprived.yaml
@@ -46,7 +46,7 @@ get:
                   minimum: 0
                   example: 0
     400:
-      description: "Common HTTP semantics, see the response body."
+      $ref: '../zoo-api.yaml#/components/responses/ReqFailure'
     401:
       $ref: '../zoo-api.yaml#/components/responses/JwtFailure'
     403:
diff --git a/zoo-api.yaml b/zoo-api.yaml
index e33a50b..8a13ee6 100644
--- a/zoo-api.yaml
+++ b/zoo-api.yaml
@@ -148,16 +148,16 @@ components:
         type: string
   responses:
     ReqFailure:
-      description: |-
-        The request isn't valid; for example, it is missing a required parameter.
-        content:
-          application/json:
+      description: The request isn't valid; for example, it is missing a required parameter.
+      content:
+        application/json:
+          schema:
             type: object
             properties:
               err:
                 description: A plain text explanation of what's wrong with the request
                 type: string
-                example: Either pu or bsn query parameter needs to be present and not blank.
+                example: Either x or y query parameter needs to be present and not blank.
     JwtFailure:
       description: |-
         The JWT token isn't valid or wasn't provided.