diff --git a/aegisub/libaegisub/common/vfr.cpp b/aegisub/libaegisub/common/vfr.cpp
index 662f57f33..05bcd684e 100644
--- a/aegisub/libaegisub/common/vfr.cpp
+++ b/aegisub/libaegisub/common/vfr.cpp
@@ -73,7 +73,7 @@ struct TimecodeRange {
 	int start;
 	int end;
 	double fps;
-	bool operator<(TimecodeRange cmp) {
+	bool operator<(TimecodeRange const& cmp) const {
 		return start < cmp.start;
 	}
 	TimecodeRange() : fps(0.) { }