diff --git a/src/image.c b/src/image.c
index 1c86168..430eaeb 100644
--- a/src/image.c
+++ b/src/image.c
@@ -18,10 +18,11 @@
 #include <magic.h>
 
 #include "main.h"
+#include "bitmap.h"
 #include "image.h"
 
 image_t *
-image_from_file(const char *path, char **errstr)
+image_from_file(const char *path, const char **errstr)
 {
   const char *mimetype;
   magic_t magic;
diff --git a/src/image.h b/src/image.h
index 585045c..d86d62d 100644
--- a/src/image.h
+++ b/src/image.h
@@ -6,6 +6,6 @@ typedef struct
 } image_t;
 
 image_t *
-image_from_file(const char *path, char **errstr);
+image_from_file(const char *path, const char **errstr);
 #endif
 #define HAS_IMAGE_H
diff --git a/src/test-image.c b/src/test-image.c
index 0225944..365cd7a 100644
--- a/src/test-image.c
+++ b/src/test-image.c
@@ -1,11 +1,12 @@
 #include <gd.h>
 
 #include "main.h"
+#include "bitmap.h"
 #include "image.h"
 
 int main(int argc, char **argv) {
   image_t *img = NULL;
-  char *err;
+  const char *err = NULL;
 
   img = image_from_file("test.png", &err);
   if (img == NULL) {