diff --git a/solidify/array.h b/solidify/array.h
index 0f4dd25..565e0f8 100644
--- a/solidify/array.h
+++ b/solidify/array.h
@@ -49,4 +49,4 @@ static inline int get_bounded(const struct array *a, int x, int y)
 }
 
 
-#endif /* ARRAY_H */
+#endif /* !ARRAY_H */
diff --git a/solidify/histo.h b/solidify/histo.h
index 7deea0b..34269d2 100644
--- a/solidify/histo.h
+++ b/solidify/histo.h
@@ -26,4 +26,4 @@ struct histo *make_histo(struct array *a);
 void free_histo(struct histo *h);
 int median(const struct histo *h);
 
-#endif /* HISTO_H */
+#endif /* !HISTO_H */
diff --git a/solidify/level.h b/solidify/level.h
index e30ae76..9a6f7f5 100644
--- a/solidify/level.h
+++ b/solidify/level.h
@@ -20,4 +20,4 @@
 
 void level(GtkWidget *canvas, struct face *f);
 
-#endif /* LEVEL_H */
+#endif /* !LEVEL_H */
diff --git a/solidify/solid.h b/solidify/solid.h
index 1e84088..086d4cc 100644
--- a/solidify/solid.h
+++ b/solidify/solid.h
@@ -21,4 +21,4 @@ struct solid {
 
 void povray(const struct solid *s);
 
-#endif /* SOLID_H */
+#endif /* !SOLID_H */
diff --git a/solidify/util.h b/solidify/util.h
index 8bcd292..933bb96 100644
--- a/solidify/util.h
+++ b/solidify/util.h
@@ -28,4 +28,4 @@
 
 void draw_circle(GdkDrawable *da, GdkGC *gc, int x, int y, int r);
 
-#endif /* UTIL_H */
+#endif /* !UTIL_H */