diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
index 7b58eb6..2fc8197 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
@@ -32,6 +32,7 @@
 import {appContext} from '../../../services/app-context';
 import {
   getParentIndex,
+  isAParent,
   isMergeParent,
   isNumber,
 } from '../../../utils/patch-set-util';
@@ -729,8 +730,26 @@
   }
 
   _handleCreateComment(e: CustomEvent) {
-    const {lineNum, side, patchNum, range, path} = e.detail;
-    const commentSide = this._sideToCommentSide(side);
+    if (!this.patchRange) throw Error('patch range not set');
+
+    const {lineNum, side, range, path} = e.detail;
+
+    // Usually, the comment is stored on the patchset shown on the side the
+    // user added the comment on, and the commentSide will be REVISION.
+    // However, if the comment is added on the left side of the diff and the
+    // version shown there is not a patchset that is part the change, but
+    // instead a base (a PARENT or a merge parent commit), the comment is
+    // stored on the patchset shown on the right, and commentSide=PARENT
+    // indicates that the comment should still be shown on the left side.
+    const patchNum =
+      side === Side.LEFT && !isAParent(this.patchRange.basePatchNum)
+        ? this.patchRange.basePatchNum
+        : this.patchRange.patchNum;
+    const commentSide =
+      side === Side.LEFT && isAParent(this.patchRange.basePatchNum)
+        ? CommentSide.PARENT
+        : CommentSide.REVISION;
+
     const threadEl = this._getOrCreateThread(
       patchNum,
       lineNum,
@@ -744,15 +763,6 @@
     this.reporting.recordDraftInteraction();
   }
 
-  _sideToCommentSide(side: Side): CommentSide {
-    if (!this.patchRange) throw Error('patch range not set');
-    return side === Side.LEFT &&
-      (this.patchRange.basePatchNum === 'PARENT' ||
-        isMergeParent(this.patchRange.basePatchNum))
-      ? CommentSide.PARENT
-      : CommentSide.REVISION;
-  }
-
   /**
    * Gets or creates a comment thread at a given location.
    * May provide a range, to get/create a range comment.
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
index ad46bcd..0c1a92d 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
@@ -966,7 +966,6 @@
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: 2,
           lineNum: 3,
           side: diffSide,
           path: '/p',
@@ -989,10 +988,13 @@
         end_line: 1,
         end_character: 3,
       };
+      element.patchRange = {
+        basePatchNum: 'PARENT',
+        patchNum: 3,
+      };
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: 3,
           lineNum: 1,
           side: diffSide,
           path: '/p',
@@ -1018,7 +1020,6 @@
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: 2,
           side: Side.RIGHT,
         },
       }));
@@ -1037,7 +1038,6 @@
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: 3,
           side: Side.LEFT,
         },
       }));
@@ -1056,7 +1056,6 @@
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: 3,
           side: Side.LEFT,
         },
       }));
@@ -1075,7 +1074,6 @@
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: 3,
           side: Side.LEFT,
         },
       }));
@@ -1097,8 +1095,6 @@
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: 2,
-          lineNum: 3,
           side: diffSide,
           path: '/p',
         },
@@ -1123,8 +1119,6 @@
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: 2,
-          lineNum: 3,
           side: diffSide,
           path: '/p',
         },
@@ -1149,11 +1143,8 @@
 
       element.dispatchEvent(new CustomEvent('create-comment', {
         detail: {
-          patchNum: 2,
-          lineNum: 3,
           side: diffSide,
           path: '/p',
-          range: undefined,
         },
       }));