--- a/src/block.h
+++ b/src/block.h
@@ -38,7 +38,7 @@
  *   The string literal ``\ No newline at end of file'' including the
  *   newline at the end.
  */
-const char *DIFFFILTER_DIFF_NO_NL_AT_EOF;
+extern const char *DIFFFILTER_DIFF_NO_NL_AT_EOF;
 
 struct diff_block;
 typedef struct diff_block *diff_block_t;