diff --git a/src/main.cpp b/src/main.cpp
index 9e2ebb932e..424e8dfdd9 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1993,7 +1993,7 @@ bool CheckDiskSpace(uint64 nAdditionalBytes)
 
 static filesystem::path BlockFilePath(unsigned int nFile)
 {
-    string strBlockFn = strprintf("blk%04d.dat", nFile);
+    string strBlockFn = strprintf("blk%04u.dat", nFile);
     return GetDataDir() / strBlockFn;
 }