diff --git a/src/validation.cpp b/src/validation.cpp
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -4303,7 +4303,7 @@
 
     fs::path path = GetBlockPosFilename(pos, prefix);
     fs::create_directories(path.parent_path());
-    FILE *file = fsbridge::fopen(path, "rb+");
+    FILE *file = fsbridge::fopen(path, fReadOnly ? "rb" : "rb+");
     if (!file && !fReadOnly) {
         file = fsbridge::fopen(path, "wb+");
     }