diff --git a/layout/base/crashtests/1729578.html b/layout/base/crashtests/1729578.html
new file mode 100644
index 000000000000..27dd8a140fe3
--- /dev/null
+++ b/layout/base/crashtests/1729578.html
@@ -0,0 +1,26 @@
+<style>
+#b {
+  column-span: all;
+}
+* {
+  column-count: 17;
+}
+.c {
+  column-span: all;
+  overflow-wrap: break-word;
+  height: 0.001592530849548246em;
+}
+</style>
+<script>
+let go = () => {
+  a.appendChild(d)
+}
+</script>
+<body onload=go()>
+<ol id="b">
+<li></li>
+</ol>
+<a id="a">x</a>
+<dl id="d">
+<dd class="c">;MLCI|=oV;nvAP*o7U</dd>
+<u>x</u>
diff --git a/layout/base/crashtests/crashtests.list b/layout/base/crashtests/crashtests.list
index 4e5e8132ba3f..3b031b4469d3 100644
--- a/layout/base/crashtests/crashtests.list
+++ b/layout/base/crashtests/crashtests.list
@@ -560,3 +560,4 @@ load 1685146.html
 load 1689912.html
 load 1690163.html
 load 1723200.html
+load 1729578.html
diff --git a/layout/generic/nsTextFrame.cpp b/layout/generic/nsTextFrame.cpp
index f2b7ed65f588..100cdd432244 100644
--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -4493,8 +4493,11 @@ class nsContinuingTextFrame final : public nsTextFrame {
 
   nsIFrame* FirstInFlow() const final;
   nsTextFrame* FirstContinuation() const final {
-    MOZ_DIAGNOSTIC_ASSERT(mFirstContinuation || !mPrevContinuation,
-                          "mFirstContinuation unexpectedly null!");
+    // If we have a prev-continuation pointer, then our first-continuation
+    // must be the same as that frame's.
+    MOZ_ASSERT((!mPrevContinuation && !mFirstContinuation) ||
+               (mPrevContinuation &&
+                mPrevContinuation->FirstContinuation() == mFirstContinuation));
     return mFirstContinuation;
   };
 
