diff --git a/updater/blockimg.cpp b/updater/blockimg.cpp
index 2e30c5b..12a549d 100644
--- a/updater/blockimg.cpp
+++ b/updater/blockimg.cpp
@@ -192,7 +192,7 @@
 }
 
 struct RangeSinkState {
-    RangeSinkState(RangeSet& rs) : tgt(rs) { };
+    explicit RangeSinkState(RangeSet& rs) : tgt(rs) { };
 
     int fd;
     const RangeSet& tgt;