diff --git a/src/interfaces/libpq/win32.mak b/src/interfaces/libpq/win32.mak
index 1d8c84d5a28..88c36570352 100644
--- a/src/interfaces/libpq/win32.mak
+++ b/src/interfaces/libpq/win32.mak
@@ -16,7 +16,7 @@ CPU=i386
 !MESSAGE Building the Win32 static library...
 !MESSAGE
 !ELSEIF ("$(CPU)" == "IA64")||("$(CPU)" == "AMD64")
-ADD_DEFINES=/D "_WIN64" /Wp64 /GS
+ADD_DEFINES=/Wp64 /GS
 ADD_SECLIB=bufferoverflowU.lib
 !MESSAGE Building the Win64 static library...
 !MESSAGE