diff --git a/c/isogram/src/isogram.h b/c/isogram/src/isogram.h
index eadfd74..ae28cfa 100644
--- a/c/isogram/src/isogram.h
+++ b/c/isogram/src/isogram.h
@@ -3,7 +3,6 @@
 
 #include <stdbool.h>
 
-int length(const char string[]);
 bool is_isogram(const char phrase[]);
 
 #endif