diff --git a/src/main.cpp b/src/main.cpp
index f5fd7561c..05f1a2eb4 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1106,7 +1106,7 @@ bool WriteBlockToDisk(CBlock& block, CDiskBlockPos& pos)
 {
     // Open history file to append
     CAutoFile fileout(OpenBlockFile(pos), SER_DISK, CLIENT_VERSION);
-    if (!fileout)
+    if (fileout.IsNull())
         return error("WriteBlockToDisk : OpenBlockFile failed");
 
     // Write index header
@@ -1134,7 +1134,7 @@ bool ReadBlockFromDisk(CBlock& block, const CDiskBlockPos& pos)
 
     // Open history file to read
     CAutoFile filein(OpenBlockFile(pos, true), SER_DISK, CLIENT_VERSION);
-    if (!filein)
+    if (filein.IsNull())
         return error("ReadBlockFromDisk : OpenBlockFile failed");
 
     // Read block
@@ -4548,7 +4548,7 @@ bool CBlockUndo::WriteToDisk(CDiskBlockPos &pos, const uint256 &hashBlock)
 {
     // Open history file to append
     CAutoFile fileout(OpenUndoFile(pos), SER_DISK, CLIENT_VERSION);
-    if (!fileout)
+    if (fileout.IsNull())
         return error("CBlockUndo::WriteToDisk : OpenUndoFile failed");
 
     // Write index header
@@ -4580,7 +4580,7 @@ bool CBlockUndo::ReadFromDisk(const CDiskBlockPos &pos, const uint256 &hashBlock
 {
     // Open history file to read
     CAutoFile filein(OpenUndoFile(pos, true), SER_DISK, CLIENT_VERSION);
-    if (!filein)
+    if (filein.IsNull())
         return error("CBlockUndo::ReadFromDisk : OpenBlockFile failed");
 
     // Read block
diff --git a/src/net.cpp b/src/net.cpp
index 50b435cf1..b7c958143 100644
--- a/src/net.cpp
+++ b/src/net.cpp
@@ -1929,7 +1929,7 @@ bool CAddrDB::Write(const CAddrMan& addr)
     boost::filesystem::path pathTmp = GetDataDir() / tmpfn;
     FILE *file = fopen(pathTmp.string().c_str(), "wb");
     CAutoFile fileout(file, SER_DISK, CLIENT_VERSION);
-    if (!fileout)
+    if (fileout.IsNull())
         return error("%s : Failed to open file %s", __func__, pathTmp.string());
 
     // Write and commit header, data
@@ -1954,7 +1954,7 @@ bool CAddrDB::Read(CAddrMan& addr)
     // open input file, and associate with CAutoFile
     FILE *file = fopen(pathAddr.string().c_str(), "rb");
     CAutoFile filein(file, SER_DISK, CLIENT_VERSION);
-    if (!filein)
+    if (filein.IsNull())
         return error("%s : Failed to open file %s", __func__, pathAddr.string());
 
     // use file size to size memory buffer
diff --git a/src/serialize.h b/src/serialize.h
index 55b689139..d8f0b76d5 100644
--- a/src/serialize.h
+++ b/src/serialize.h
@@ -1122,7 +1122,7 @@ public:
     FILE& operator*()           { return *file; }
     FILE** operator&()          { return &file; }
     FILE* operator=(FILE* pnew) { return file = pnew; }
-    bool operator!()            { return (file == NULL); }
+    bool IsNull() const         { return (file == NULL); }
 
 
     //