diff --git a/include/project.h b/include/project.h
index e6e48c1..8fd34c8 100644
--- a/include/project.h
+++ b/include/project.h
@@ -37,5 +37,7 @@
 #define PROG_RET_UNMAP_HAYSTACK_N_FAILED(h) (((h) << 2) | 1)
 // Unmapping of argv[1] failed
 #define PROG_RET_UNMAP_NEEDLE_FAILED 3
+// Internal error
+#define PROG_RET_INTERNAL -1
 
 #endif /* _PROJECT_H */