diff --git a/layout/base/nsLayoutDebugger.cpp b/layout/base/nsLayoutDebugger.cpp
index 317a2d2b470b..565cf262f24e 100644
--- a/layout/base/nsLayoutDebugger.cpp
+++ b/layout/base/nsLayoutDebugger.cpp
@@ -129,6 +129,7 @@ static void PrintDisplayItemTo(nsDisplayListBuilder* aBuilder,
     }
 
     MOZ_ASSERT_UNREACHABLE();
+    return "";
   };
 
   aStream << nsPrintfCString(" reuse-state(%s)",