diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
index ef22188..5d32e9b 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
@@ -815,10 +815,9 @@
         // for each line from the start.
         let lastEl;
         for (const threadEl of addedThreadEls) {
-          const lineNumString = threadEl.getAttribute('line-num') || 'FILE';
           const commentSide = threadEl.getAttribute('comment-side');
-          const lineEl = this.$.diffBuilder.getLineElByNumber(
-              lineNumString, commentSide);
+          const lineEl = this._getLineElement(threadEl,
+              commentSide);
           const contentText = this.$.diffBuilder.getContentByLineEl(lineEl);
           const contentEl = contentText.parentElement;
           const threadGroupEl = this._getOrCreateThreadGroup(
@@ -844,6 +843,18 @@
       });
     }
 
+    _getLineElement(threadEl, commentSide) {
+      const lineNumString = threadEl.getAttribute('line-num') || 'FILE';
+      const lineEl = this.$.diffBuilder.getLineElByNumber(
+          lineNumString, commentSide);
+      if (lineEl) {
+        return lineEl;
+      }
+      // It is possible to add comment to non-existing line via API
+      threadEl.invalidLineNumber = true;
+      return this.$.diffBuilder.getLineElByNumber('FILE', commentSide);
+    }
+
     _unobserveIncrementalNodes() {
       if (this._incrementalNodeObserver) {
         Polymer.dom(this).unobserveNodes(this._incrementalNodeObserver);
diff --git a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.html b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.html
index cce1d19..7bc93b5 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.html
+++ b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.html
@@ -16,6 +16,8 @@
 -->
 
 <link rel="import" href="/bower_components/polymer/polymer.html">
+<link rel="import" href="/bower_components/iron-icon/iron-icon.html">
+<link rel="import" href="../../shared/gr-icons/gr-icons.html">
 <link rel="import" href="../../../behaviors/fire-behavior/fire-behavior.html">
 <link rel="import" href="../../../behaviors/gr-path-list-behavior/gr-path-list-behavior.html">
 <link rel="import" href="../../../styles/shared-styles.html">
@@ -78,6 +80,18 @@
         margin-left: var(--spacing-m);
         font-style: italic;
       }
+      .invalidLineNumber {
+        padding: var(--spacing-m);
+      }
+      .invalidLineNumberText {
+        color: var(--error-text-color);
+      }
+      .invalidLineNumberIcon {
+        color: var(--error-text-color);
+        vertical-align: top;
+        margin-right: var(--spacing-s);
+      }
+
     </style>
     <template is="dom-if" if="[[showFilePath]]">
       <div class="pathInfo">
@@ -86,6 +100,11 @@
       </div>
     </template>
     <div id="container" class$="[[_computeHostClass(unresolved, isRobotComment)]]">
+      <template is="dom-if" if="[[invalidLineNumber]]">
+        <div class="invalidLineNumber">
+          <span class="invalidLineNumberText"><iron-icon icon="gr-icons:error" class="invalidLineNumberIcon"></iron-icon>This comment thread is attached to non-existing line [[lineNum]].</span>
+        </div>
+      </template>
       <template id="commentList" is="dom-repeat" items="[[_orderedComments]]"
           as="comment">
         <gr-comment
diff --git a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.js b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.js
index d8a56f8..00ff03a 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.js
+++ b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.js
@@ -124,6 +124,11 @@
           type: Boolean,
           value: false,
         },
+        /** It is possible to add comment to non-existing line via API */
+        invalidLineNumber: {
+          type: Number,
+          reflectToAttribute: true,
+        },
         /** Necessary only if showFilePath is true or when used with gr-diff */
         lineNum: {
           type: Number,
diff --git a/polygerrit-ui/app/elements/shared/gr-icons/gr-icons.html b/polygerrit-ui/app/elements/shared/gr-icons/gr-icons.html
index 743923b..5ecae00 100644
--- a/polygerrit-ui/app/elements/shared/gr-icons/gr-icons.html
+++ b/polygerrit-ui/app/elements/shared/gr-icons/gr-icons.html
@@ -54,6 +54,8 @@
       <g id="info"><path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm1 15h-2v-6h2v6zm0-8h-2V7h2v2z"></path></g>
       <!-- This SVG is a copy from material.io https://material.io/icons/#ic_hourglass_full-->
       <g id="hourglass"><path d="M6 2v6h.01L6 8.01 10 12l-4 4 .01.01H6V22h12v-5.99h-.01L18 16l-4-4 4-3.99-.01-.01H18V2H6z"/><path d="M0 0h24v24H0V0z" fill="none"/></g>
+      <!-- This SVG is a copy from iron-icons https://github.com/PolymerElements/iron-icons/blob/master/iron-icons.js -->
+      <g id="error"><path d="M12 2C6.48 2 2 6.48 2 12s4.48 10 10 10 10-4.48 10-10S17.52 2 12 2zm1 15h-2v-2h2v2zm0-4h-2V7h2v6z"></path></g>
       <!-- This is a custom PolyGerrit SVG -->
       <g id="side-by-side"><path d="M17.1578947,10.8888889 L2.84210526,10.8888889 C2.37894737,10.8888889 2,11.2888889 2,11.7777778 L2,17.1111111 C2,17.6 2.37894737,18 2.84210526,18 L17.1578947,18 C17.6210526,18 18,17.6 18,17.1111111 L18,11.7777778 C18,11.2888889 17.6210526,10.8888889 17.1578947,10.8888889 Z M17.1578947,2 L2.84210526,2 C2.37894737,2 2,2.4 2,2.88888889 L2,8.22222222 C2,8.71111111 2.37894737,9.11111111 2.84210526,9.11111111 L17.1578947,9.11111111 C17.6210526,9.11111111 18,8.71111111 18,8.22222222 L18,2.88888889 C18,2.4 17.6210526,2 17.1578947,2 Z M16.1973628,2 L2.78874238,2 C2.35493407,2 2,2.4 2,2.88888889 L2,8.22222222 C2,8.71111111 2.35493407,9.11111111 2.78874238,9.11111111 L16.1973628,9.11111111 C16.6311711,9.11111111 16.9861052,8.71111111 16.9861052,8.22222222 L16.9861052,2.88888889 C16.9861052,2.4 16.6311711,2 16.1973628,2 Z" id="Shape" transform="scale(1.2) translate(10.000000, 10.000000) rotate(-90.000000) translate(-10.000000, -10.000000)"/></g>
       <!-- This is a custom PolyGerrit SVG -->