diff --git a/src/diff.c b/src/diff.c
index 69ba7a3..02f1c33 100644
--- a/src/diff.c
+++ b/src/diff.c
@@ -712,7 +712,7 @@
 
     // xdiff requires one big block of memory with all the text.
     for (lnum = 1; lnum <= buf->b_ml.ml_line_count; ++lnum)
-	len += STRLEN(ml_get_buf(buf, lnum, FALSE)) + 1;
+	len += (long)STRLEN(ml_get_buf(buf, lnum, FALSE)) + 1;
     ptr = lalloc(len, TRUE);
     if (ptr == NULL)
     {