diff --git a/internal/debug/api.go b/internal/debug/api.go
index 3118777eed..232834d8c0 100644
--- a/internal/debug/api.go
+++ b/internal/debug/api.go
@@ -191,9 +191,9 @@ func (*HandlerT) WriteMemProfile(file string) error {
 
 // Stacks returns a printed representation of the stacks of all goroutines.
 func (*HandlerT) Stacks() string {
-	var b bytes.Buffer
-	pprof.Lookup("goroutine").WriteTo(w, 2)
-	return b.String()
+	var buf bytes.Buffer
+	pprof.Lookup("goroutine").WriteTo(buf, 2)
+	return buf.String()
 }
 
 // FreeOSMemory returns unused memory to the OS.