diff --git a/src/libsodium/include/sodium/private/common.h b/src/libsodium/include/sodium/private/common.h
index a3983818..ed026909 100644
--- a/src/libsodium/include/sodium/private/common.h
+++ b/src/libsodium/include/sodium/private/common.h
@@ -234,6 +234,7 @@ xor_buf(unsigned char *out, const unsigned char *in, size_t n)
 #endif
 
 #ifdef HAVE_LIBCTGRIND
+extern void ct_poison(const void *, size_t);
 # define POISON(X, L) ct_poison((X), (L))
 #else
 # define POISON(X, L) (void) 0