diff --git a/src/profile-logic/process-profile.ts b/src/profile-logic/process-profile.ts
index 24846ec17b..d83c75a447 100644
--- a/src/profile-logic/process-profile.ts
+++ b/src/profile-logic/process-profile.ts
@@ -73,6 +73,7 @@ import type {
   GeckoMetaMarkerSchema,
   GeckoStaticFieldSchemaData,
   GeckoMarkers,
+  GeckoMarkerStack,
   GeckoMarkerStruct,
   GeckoMarkerTuple,
   GeckoFrameStruct,
@@ -562,6 +563,20 @@ function _processStackTable(
   return stackIndexOffset;
 }
 
+/**
+ * We expect a captured backtrace here, with a samples table. But the "stack" key
+ * isn't reserved for that: some markers store an unrelated value (e.g. Log markers
+ * from the test harness put a textual stack trace string there). Such a value has
+ * no `samples`, so checking for it both selects real backtraces and keeps one bad
+ * marker from failing the whole profile. A non-backtrace stack is left in place to
+ * be handled by the marker schema like any other field.
+ */
+function _payloadHasStack(
+  data: MarkerPayload_Gecko
+): data is MarkerPayload_Gecko & { stack: GeckoMarkerStack } {
+  return 'stack' in data && !!data.stack?.samples?.data.length;
+}
+
 /**
  * Convert stack field to cause field for the given payload. A cause field includes
  * the thread ID (tid), an IndexIntoStackTable, and the time the stack was captured.
@@ -573,7 +588,7 @@ function _convertStackToCause(
   data: MarkerPayload_Gecko,
   stackIndexOffset: IndexIntoStackTable
 ) {
-  if ('stack' in data && data.stack && data.stack.samples.data.length > 0) {
+  if (_payloadHasStack(data)) {
     const { stack, ...newData } = data;
     const stackIndex = stack.samples.data[0][stack.samples.schema.stack];
     const time = stack.samples.data[0][stack.samples.schema.time];
@@ -599,7 +614,7 @@ function _convertPayloadStackToIndex(
   if (!data) {
     return null;
   }
-  if ('stack' in data && data.stack && data.stack.samples.data.length > 0) {
+  if (_payloadHasStack(data)) {
     const { samples } = data.stack;
     const geckoStackIndex = samples.data[0][samples.schema.stack];
     if (geckoStackIndex !== null) {
diff --git a/src/test/unit/marker-data.test.ts b/src/test/unit/marker-data.test.ts
index 0e49044eb5..80c8c41ab0 100644
--- a/src/test/unit/marker-data.test.ts
+++ b/src/test/unit/marker-data.test.ts
@@ -100,6 +100,44 @@ describe('Derive markers from Gecko phase markers', function () {
     ]);
   });
 
+  it('keeps a non-backtrace string stack instead of crashing', function () {
+    // The Gecko "stack" key isn't reserved for captured backtraces. Log markers
+    // emitted by the test harness store a textual JS stack trace string there.
+    // Such a value has no samples table, so it must be left untouched (not turned
+    // into a cause) rather than crashing the whole profile during processing.
+    const stackString =
+      '_abort_failed_test@head.js:938:20\ndo_report_result@head.js:1053:5\n';
+    const { markers } = setupWithTestDefinedMarkers([
+      {
+        startTime: 5,
+        endTime: null,
+        phase: INSTANT,
+        data: {
+          type: 'Log',
+          level: 1,
+          message: 'Unexpected exception NS_ERROR_ABORT',
+          stack: stackString,
+        } as any,
+      },
+    ]);
+
+    expect(markers).toEqual([
+      {
+        category: 0,
+        data: {
+          type: 'Log',
+          level: 1,
+          message: 'Unexpected exception NS_ERROR_ABORT',
+          stack: stackString,
+        },
+        end: null,
+        name: 'TestDefinedMarker',
+        start: 5,
+        threadId: null,
+      },
+    ]);
+  });
+
   it('matches an IntervalStart and IntervalEnd marker', function () {
     const { markers } = setupWithTestDefinedMarkers([
       {