diff --git a/win32/Make.rules.mak b/win32/Make.rules.mak
index 6ae58e20..da267fce 100644
--- a/win32/Make.rules.mak
+++ b/win32/Make.rules.mak
@@ -53,7 +53,7 @@ CNGSDK_INCL_DIR = "/IC:\Program Files\Microsoft CNG Development Kit\Include"
 # INTTYPES_INCL_DIR =  /IC:\opensc\dependencies\msys\local
 
 ALL_INCLUDES = /I$(TOPDIR)\win32 /I$(TOPDIR)\src $(OPENSSL_INCL_DIR) $(ZLIB_INCL_DIR) $(LIBLTDL_INCL) $(INTTYPES_INCL_DIR) $(CNGSDK_INCL_DIR)
-COPTS =  /Wall /D_CRT_SECURE_NO_DEPRECATE /MT /nologo /DHAVE_CONFIG_H $(ALL_INCLUDES) /D_WIN32_WINNT=0x0502 /DWIN32_LEAN_AND_MEAN $(OPENSSL_DEF) $(ZLIB_DEF) $(MINIDRIVER_DEF) /DOPENSC_FEATURES="\"$(OPENSC_FEATURES)\""
+COPTS =  /W4 /D_CRT_SECURE_NO_DEPRECATE /MT /nologo /DHAVE_CONFIG_H $(ALL_INCLUDES) /D_WIN32_WINNT=0x0502 /DWIN32_LEAN_AND_MEAN $(OPENSSL_DEF) $(ZLIB_DEF) $(MINIDRIVER_DEF) /DOPENSC_FEATURES="\"$(OPENSC_FEATURES)\""
 !IF "$(BUILD_FOR)" == "WIN64"
 LINKFLAGS = /NOLOGO /INCREMENTAL:NO /MACHINE:X64 /MANIFEST:NO /NODEFAULTLIB:MSVCRTD  /NODEFAULTLIB:MSVCRT /NODEFAULTLIB:LIBCMTD
 LIBFLAGS =  /nologo /machine:x64