diff --git a/vmdk/diskinfo.h b/vmdk/diskinfo.h
index 2b58f43..f4f9441 100644
--- a/vmdk/diskinfo.h
+++ b/vmdk/diskinfo.h
@@ -35,7 +35,7 @@ struct DiskInfo {
 	const DiskInfoVMT *vmt;
 };
 
-char *toolsVersion; /* toolsVersion in metadata */
+extern char *toolsVersion; /* toolsVersion in metadata */
 
 DiskInfo *Flat_Open(const char *fileName);
 DiskInfo *Flat_Create(const char *fileName, off_t capacity);
diff --git a/vmdk/mkdisk.c b/vmdk/mkdisk.c
index b6c0cc9..b66b7d5 100644
--- a/vmdk/mkdisk.c
+++ b/vmdk/mkdisk.c
@@ -24,6 +24,8 @@
 #include <string.h>
 #include <getopt.h>
 
+char *toolsVersion; /* toolsVersion in metadata */
+
 static int
 copyData(DiskInfo *dst,
          off_t dstOffset,