diff --git a/tools/c-index-test/c-index-test.c b/tools/c-index-test/c-index-test.c
index 773c009..84ceab2 100644
--- a/tools/c-index-test/c-index-test.c
+++ b/tools/c-index-test/c-index-test.c
@@ -109,7 +109,7 @@
     }
 
     /* Open the file that we're remapping to. */
-    to_file = fopen(semi + 1, "r");
+    to_file = fopen(semi + 1, "rb");
     if (!to_file) {
       fprintf(stderr, "error: cannot open file %s that we are remapping to\n",
               semi + 1);