diff --git a/layout/generic/nsBlockFrame.cpp b/layout/generic/nsBlockFrame.cpp
index 8abd928ce55..c6166d8d9d5 100644
--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -1923,7 +1923,7 @@ nsBlockFrame::ReflowDirtyLines(nsBlockReflowState& aState)
       // to the result of nextLine->ShouldApplyTopMargin.
       if (line.next() != end_lines()) {
         PRBool maybeWasEmpty = oldY == line.next()->mBounds.y;
-        PRBool isEmpty = line->mBounds.height == 0 && line->CachedIsEmpty();
+        PRBool isEmpty = line->mBounds.width == 0 && line->CachedIsEmpty();
         if (maybeReflowingForFirstTime /*1*/ ||
             (isEmpty || maybeWasEmpty) /*2/3/4*/) {
           line.next()->MarkPreviousMarginDirty();
diff --git a/layout/reftests/bugs/418766-1-ref.html b/layout/reftests/bugs/418766-1-ref.html
new file mode 100644
index 00000000000..a205c5a4950
--- /dev/null
+++ b/layout/reftests/bugs/418766-1-ref.html
@@ -0,0 +1,5 @@
+<html>
+<body>
+  b
+</body>
+</html>
diff --git a/layout/reftests/bugs/418766-1a.html b/layout/reftests/bugs/418766-1a.html
new file mode 100644
index 00000000000..4187b8b10ac
--- /dev/null
+++ b/layout/reftests/bugs/418766-1a.html
@@ -0,0 +1,16 @@
+<html class="reftest-wait">
+<head>
+<script>
+function boom()
+{
+  var n = document.getElementById("editMe");
+  n.removeChild(n.firstChild);
+  document.documentElement.className = "";
+}
+</script>
+</head>
+<body onload="boom();">
+  <span id="editMe" style="margin-bottom: 1px;">a</span>
+  <div>b</div>
+</body>
+</html>
diff --git a/layout/reftests/bugs/418766-1b.html b/layout/reftests/bugs/418766-1b.html
new file mode 100644
index 00000000000..ded8ea9eded
--- /dev/null
+++ b/layout/reftests/bugs/418766-1b.html
@@ -0,0 +1,6 @@
+<html>
+<body>
+  <span id="editMe" style="margin-bottom: 1px;"></span>
+  <div>b</div>
+</body>
+</html>
diff --git a/layout/reftests/bugs/reftest.list b/layout/reftests/bugs/reftest.list
index 50a9a837492..8bd9cae87ed 100644
--- a/layout/reftests/bugs/reftest.list
+++ b/layout/reftests/bugs/reftest.list
@@ -738,6 +738,8 @@ random == 403134-1.html 403134-1-ref.html # bug 405377
 == 416106-1.xhtml 416106-1-ref.xhtml
 == 416752-1.html 416752-1-ref.html
 == 417676.html 417676-ref.html
+== 418766-1a.html 418766-1-ref.html
+== 418766-1b.html 418766-1-ref.html
 == 419060.html 419060-ref.html
 == 419531-1.html 419531-1-ref.html
 == 420069-1.html 420069-1-ref.html
