diff --git a/keyserver/src/fetchers/message-fetchers.js b/keyserver/src/fetchers/message-fetchers.js
--- a/keyserver/src/fetchers/message-fetchers.js
+++ b/keyserver/src/fetchers/message-fetchers.js
@@ -721,13 +721,9 @@
 > {
   const requiredIDs = new Set<string>();
   for (const row of rows) {
-    if (row.type === messageTypes.SIDEBAR_SOURCE) {
-      const content = JSON.parse(row.content);
-      requiredIDs.add(content.sourceMessageID);
-    } else if (row.type === messageTypes.TOGGLE_PIN) {
-      const content = JSON.parse(row.content);
-      requiredIDs.add(content.targetMessageID);
-    }
+    // parseDerivedMessages should be defined for SIDEBAR_SOURCE and TOGGLE_PIN
+    const { parseDerivedMessages } = messageSpecs[row.type];
+    parseDerivedMessages?.(row, requiredIDs);
   }
 
   const messagesByID = new Map<
diff --git a/lib/shared/messages/message-spec.js b/lib/shared/messages/message-spec.js
--- a/lib/shared/messages/message-spec.js
+++ b/lib/shared/messages/message-spec.js
@@ -116,6 +116,7 @@
   +canBeSidebarSource: boolean,
   +canBePinned: boolean,
   +canBeRenderedIndependently?: boolean,
+  +parseDerivedMessages?: (row: Object, requiredIDs: Set<string>) => void,
   +useCreationSideEffectsFunc?: () => CreationSideEffectsFunc<RawInfo>,
   +validator: TType<RawInfo>,
 };
diff --git a/lib/shared/messages/sidebar-source-message-spec.js b/lib/shared/messages/sidebar-source-message-spec.js
--- a/lib/shared/messages/sidebar-source-message-spec.js
+++ b/lib/shared/messages/sidebar-source-message-spec.js
@@ -176,5 +176,16 @@
 
   canBePinned: false,
 
+  parseDerivedMessages(row: Object, requiredIDs: Set<string>): void {
+    try {
+      const content = JSON.parse(row.content);
+      requiredIDs.add(content.sourceMessageID);
+    } catch (e) {
+      console.error(
+        `Error parsing content of message with id ${row.id}: ${e.message}`,
+      );
+    }
+  },
+
   validator: rawSidebarSourceMessageInfoValidator,
 });
diff --git a/lib/shared/messages/toggle-pin-message-spec.js b/lib/shared/messages/toggle-pin-message-spec.js
--- a/lib/shared/messages/toggle-pin-message-spec.js
+++ b/lib/shared/messages/toggle-pin-message-spec.js
@@ -159,5 +159,16 @@
 
   canBePinned: false,
 
+  parseDerivedMessages(row: Object, requiredIDs: Set<string>): void {
+    try {
+      const content = JSON.parse(row.content);
+      requiredIDs.add(content.targetMessageID);
+    } catch (e) {
+      console.error(
+        `Error parsing content of message with id ${row.id}: ${e.message}`,
+      );
+    }
+  },
+
   validator: rawTogglePinMessageInfoValidator,
 });