diff --git a/lib/selectors/chat-selectors.js b/lib/selectors/chat-selectors.js
--- a/lib/selectors/chat-selectors.js
+++ b/lib/selectors/chat-selectors.js
@@ -270,6 +270,7 @@
 
 export type RobotextChatMessageInfoItem = {
   +itemType: 'message',
+  +messageInfoType: 'robotext',
   +messageInfo: RobotextMessageInfo,
   +startsConversation: boolean,
   +startsCluster: boolean,
@@ -282,6 +283,7 @@
   | RobotextChatMessageInfoItem
   | {
       +itemType: 'message',
+      +messageInfoType: 'composable',
       +messageInfo: ComposableMessageInfo,
       +localMessageInfo: ?LocalMessageInfo,
       +startsConversation: boolean,
@@ -458,6 +460,7 @@
         messageStore.local[messageKey(originalMessageInfo)];
       chatMessageItems.push({
         itemType: 'message',
+        messageInfoType: 'composable',
         messageInfo: originalMessageInfo,
         localMessageInfo,
         startsConversation,
@@ -479,6 +482,7 @@
       );
       chatMessageItems.push({
         itemType: 'message',
+        messageInfoType: 'robotext',
         messageInfo: originalMessageInfo,
         startsConversation,
         startsCluster,
diff --git a/native/chat/chat-item-height-measurer.react.js b/native/chat/chat-item-height-measurer.react.js
--- a/native/chat/chat-item-height-measurer.react.js
+++ b/native/chat/chat-item-height-measurer.react.js
@@ -120,25 +120,30 @@
           contentHeight: height,
           reactions: item.reactions,
         };
-      } else {
-        invariant(
-          typeof item.robotext === 'string',
-          "Flow can't handle our fancy types :(",
-        );
-        return {
-          itemType: 'message',
-          messageShapeType: 'robotext',
-          messageInfo,
-          threadInfo,
-          startsConversation: item.startsConversation,
-          startsCluster: item.startsCluster,
-          endsCluster: item.endsCluster,
-          threadCreatedFromMessage: item.threadCreatedFromMessage,
-          robotext: item.robotext,
-          contentHeight: height,
-          reactions: item.reactions,
-        };
       }
+      invariant(
+        item.messageInfoType !== 'composable',
+        'ChatItemHeightMeasurer was handed a messageInfoType=composable, but ' +
+          `does not know how to handle MessageType ${messageInfo.type}`,
+      );
+      invariant(
+        item.messageInfoType === 'robotext',
+        'ChatItemHeightMeasurer was handed a messageInfoType that it does ' +
+          `not recognize: ${item.messageInfoType}`,
+      );
+      return {
+        itemType: 'message',
+        messageShapeType: 'robotext',
+        messageInfo,
+        threadInfo,
+        startsConversation: item.startsConversation,
+        startsCluster: item.startsCluster,
+        endsCluster: item.endsCluster,
+        threadCreatedFromMessage: item.threadCreatedFromMessage,
+        robotext: item.robotext,
+        contentHeight: height,
+        reactions: item.reactions,
+      };
     },
     [composedMessageMaxWidth, inputStatePendingUploads, threadInfo],
   );