diff --git a/lib/config-win32.h b/lib/config-win32.h
index b37f5525796389143bff92374c1a912594f1382a..63847426d140d5160b05bf9e2cdacbdbfd672291 100644
--- a/lib/config-win32.h
+++ b/lib/config-win32.h
@@ -31,6 +31,9 @@
 #define socklen_t int
 #endif
 
+/* The size of a `curl_off_t', as computed by sizeof. */
+#define SIZEOF_CURL_OFF_T 8
+
 /* Define if you have the ANSI C header files.  */
 #define STDC_HEADERS 1