diff --git a/tildes/scss/modules/_comment.scss b/tildes/scss/modules/_comment.scss
index 5d0a8f3..0ebd03f 100644
--- a/tildes/scss/modules/_comment.scss
+++ b/tildes/scss/modules/_comment.scss
@@ -242,27 +242,33 @@
 
 @mixin flatten-single-replies-above-depth($depth) {
   // Makes it so that comments of the specified depth or greater will not indent
-  // their replies unless there is more than one reply. Instead, it adds text at the
-  // top of the next comment (the single reply) indicating that it's a direct reply to
-  // the above comment.
+  // their replies when they only have one reply *and* they're also the only reply to
+  // their parent (this avoids confusion between subtrees and siblings).
+  // When indenting is avoided, it adds text at the top of the next comment (the single
+  // reply) indicating that it's a direct reply to the above comment.
 
-  // We actually need the selector to check 1 level up from the specified starting
+  // We actually need the selector to check 2 levels up from the specified starting
   // depth because of how we're utilizing nesting.
-  $depth: $depth - 1;
+  @if ($depth < 2) {
+    @error "Starting depth must be at least 2";
+  }
+  $depth: $depth - 2;
 
   .comment[data-comment-depth="#{$depth}"] {
-    .comment[data-comment-replies="1"] {
-      & > .comment-tree-replies {
-        margin-left: -1px;  // compensate for border
-
-        & > .comment-tree-item > .comment > .comment-itself {
-          & > .comment-text::before,
-          & > header > .is-comment-deleted::before,
-          & > header > .is-comment-removed::before {
-            content: "(Reply to above comment)";
-            font-size: 0.6rem;
-            font-style: italic;
-            margin-right: 0.2rem;
+    .comment[data-comment-replies="1"] > .comment-tree-replies > .comment-tree-item {
+      & > .comment[data-comment-replies="1"] {
+        & > .comment-tree-replies {
+          margin-left: -1px;  // compensate for border
+
+          & > .comment-tree-item > .comment > .comment-itself {
+            & > .comment-text::before,
+            & > header > .is-comment-deleted::before,
+            & > header > .is-comment-removed::before {
+              content: "(Reply to above comment)";
+              font-size: 0.6rem;
+              font-style: italic;
+              margin-right: 0.2rem;
+            }
           }
         }
       }